BMEP is a good indicator for comparisons of different engine types since it is essentially displacement-normalized torque. It is entirely possible to have diesel engines with comparable BMEP to gasoline engines and BMEP is trending upwards on both types of engines in the U.S. as they seek to improve efficiency and comply with new CO2 limits. There are turbocharged GDI SI gasoline engines with 21-23 bar BMEP and with peak BMEP available across a broad range of rpm. There are also 21-23 bar BMEP turbocharged, DI diesel engines although these are more common in heavy-duty applications than in light-duty applicaitons. Rated power is somewhat less because maximum RPM is less. There is insufficient time for a sufficient mass-fraction-burn for diffusional diesel combustion to achieve comparable rated-engine-speed to spark-ignited, homogenous charge combustion, so rated engine speed is lower and peak rated power is lower even though peak BMEP can be comparable or higher.

Dear Vishal,

To understand why petrol engines give more power than diesel engine, try to plot in the same diagram the variations of pressure with respect to volume for the same compression rate. You will visualize that the area is bigger for the petrol engine.

I would like to add that generally diesel engines produce more torque than a petrol equivalent. This chart gives some of the properties for ethanol, diesel and petrol. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&sqi=2&ved=0CCwQFjAA&url=http%3A%2F%2Fcta.ornl.gov%2Fbedb%2Fbiofuels%2Fethanol%2FFuel_Property_Comparison_for_Ethanol-Gasoline-No2Diesel.xls&ei=gKWUUo_iE8er2AWJwIHgCQ&usg=AFQjCNFycRxvpRGpLmFos0VnRrS8WGK_dw&sig2=VPYFGptXb8vQnoky6KrOgQ&bvm=bv.57155469,d.b2I

BMEP is a good indicator for comparisons of different engine types since it is essentially displacement-normalized torque. It is entirely possible to have diesel engines with comparable BMEP to gasoline engines and BMEP is trending upwards on both types of engines in the U.S. as they seek to improve efficiency and comply with new CO2 limits. There are turbocharged GDI SI gasoline engines with 21-23 bar BMEP and with peak BMEP available across a broad range of rpm. There are also 21-23 bar BMEP turbocharged, DI diesel engines although these are more common in heavy-duty applications than in light-duty applicaitons. Rated power is somewhat less because maximum RPM is less. There is insufficient time for a sufficient mass-fraction-burn for diffusional diesel combustion to achieve comparable rated-engine-speed to spark-ignited, homogenous charge combustion, so rated engine speed is lower and peak rated power is lower even though peak BMEP can be comparable or higher.

Power=Torque*Speed.

A diesel uses compression ignition, its compression ratio (e.g, 16:1) is much higher than that of a petrol engine (e.g, 10:1). As an effect, a gasoline engine runs much higher speed than that of a diesel.

There are many ways to compare the two engine types (same speed, same compression ratio, same fuel energy input). I think Joseph McDonald's response was a good one in that some normalizing measure should be used in the comparison.

The main reason is the higher rpm of petrol engine compared to diesel engine. We know that the effective power is equal to torque times angular speed. Although diesel engines produce more torque, the the engine speed is more dominant than torque. The higher speeds of petrol engines are mainly based on ignition technique. In petrol engine ignition started by a spark plug. Thus ignition retard is very small. In diesel engine ignition starts by self ignition and ignition retard is highyl greater than petrol engine. This makes the petrol engines more speedy.

The other but minor reason is heating value of the fuel. Petrol has higher heating value than diesel fuel. I hope this explanations give sufficient knowledge about this subject.

The efficiency is mainly depend on compression ratio. We know that diesel engine has highly over compression ratios than petrol engies. This makes diesel engines more efficient than petrol engines.

I think your question should be more specific; like "For the same displacement volume, why does a 4S petrol engine develops greater power compared to 4S diesel engine?".

Assuming that you asked that question, let us look into the factors that might have affected the power ouput. [I should warn you that I myself do not know the answer, but I guess I can help you to find a way or do math]

1) As you guessed, the calorific value _might_ be one of the reasons. Petrol has a greater calorific value than diesel per unit mass, but again it is a few 1000 kJ which, I believe, is not going to create much difference. [Do this math: Find the brake specific energy consumption of the two given engines - if you have experimental data - and compare the data]

2) On the thermal efficiency front, diesel engine fares better and we should be expecting a greater power output for diesel engine. If you have the experimental data for the two engines of mass-flow rate at a given load condition (for same speed), compute the fuel consumed by engine in one second and multiply by the respective calorific values and compare the results. This again - according to my personal understanding - is not going to be significantly different.

3) The third and the last factor, which I think might be reason is that diesel engines are lean burn engines. They run on leaner fuel-air mixture than petrol engines. As a result the brake mean effective pressure can be lower for diesel engine and hence the torque. There is also a practical difficulty to richen the mixture in a diesel engine [remember that diesel engines are qualitatively governed and petrol engines are quantitatively governed] after a particular limit due to smoke formation. And for a petrol engine there are no particular limits except for the materials. I, personally think, that this might be the greatest contributing factor in determining the power output. [You can again do a math if you know the stoichiometric ratios from the experimental data - i.e. air and fuel flow rates per cycle and then thermal efficiency]

It is not related to the petrol/diesel fuel composition. They are very similar in heat release. The reason the fuel inducted spark ignition engine gasoline engine has higher specific power than a compression ignition diesel engine is because of the amount of fuel in the cylinder. A compression ignition engine is essentially always at WOT conditions and power is regulated by the amount of fuel injected into the cylinder. As the power demands increase, the duration of the fuel injection process decreases often extending into the power stroke of the engine. The fuel in a compression ignition engine must fist vaporize and then burn within the cylinder. As the fuel injection process extends far into the power stroke to increase the power the atomization of the fuel is more difficult and incomplete combustion results limiting power. This is why you used to see black smoke emanating from the exhaust system in the old compression ignition engines under conditions of high power demands such as climbing hills or accelerating onto the interstates. A spark ignition engine regulated power by throttling the engine. Under high power demands or WOT the vaporization the fuel atomization is not an issue and for a port injected engine the fuel will be completed vaporized on the compression stroke, ready to combust at the time of ignition by the spark plug

There was a typo in my answer, "As the power demands increase, the duration of the fuel injection process decreases often extending into the power stroke of the engine" Should read " As the power demands increase, the duration of the fuel injection process increases often extending into the power stroke of the engine

Comparison of the power development depends on various parameters .After going through all the valid above answers I would like to add that a petrol engine is a quantity governed one and diesel is quality governed one . Due to instantaneous entry of more charge which is homogeneously mixed ,particularly during starting or in acceleration ( only more fuel in diesel engine)the rate of burning is rapid and it develops more power during first few degrees of crank angle .This generates a feeling that the petrol engine is more powerful than Diesel engine.

The efficiency of diesel engine is higher than Petrol engine obviously because of the High compression ration adopted in it. Instead of petrol &diesel composition the better way to think and compare ii it simply basing on the calorific value of both fuels.

Torque is proportional to fuel provided (,or more precisely, burnt) per revolution and power is proportional to fuel provided per unit time. Calorific values are slightly different between gasolene and diesel fuel, and thermal efficiencies are also different between two types of engines. But they are not the major reaosns that spark ignited petrol/gasolene engines and diesel engines differ in power ratings in commercial products. I believe it is possible to generate the same amount of power for the same displacement volume and the same speed from the two types of the engines as long as, say, similar amount of fuel and necessary amount of fuel can be provided and it can be done theoretically. The reason that spark ignited petrol/gasolene engines most of the cases produce higher power came from down-to-the-earth economical reasons. Not something related to the first principles of thermodynamics or chemistry. You can burn diesel fuel with compression ignition type at quite close to stoichiometry nowadays with the help of high pressure common rail system, 2500 to 3000 bar injection pressure. And whether spark ignited or compression ignited, the rest of fulel burning needs to be helped by turbulent motion of fuel-air mixture inside the cylinders, which should not be that different between the two combustion proceeding processes. Therefore, required air (which is one part of fuels) amount and required hydrocarbon amount to produce the same power are not that different between the two, althouh not exactly the same. Inherently diesel combustion (compression ignition) system requires higher compression ratio to get high temperature. (Yes, some may say there are low compression ratio diesel engines, but they require cost demanding ignition support systems.) Higher compression means more robust cylinder components, cylinder blocks, cylinder heads, cylinder gaskets, cylinder head bolts, pistons, piston rings, gudgeon pins, connecting rods, crankshafts, bearings and so on, are required. This means expensive (actually can become outrageously expensive to be reasonable commercial products) in one regard and more challenging to achieve higher speed in the other regard. Lower speed limit means lower power for the same amount of fuel amount per revolution. Unless you are a racing driver, you usually do not use the listed power in your daily driving, economical diesel engines have found their places. Having said that producing 1,500 ps (or hp) out of 1.5 liter engine, which Honda did for F1, is probably quite a challenge for a compression ignition engine.

There are a number of reasons. The main reason is that Diesel engines attain higher pressures, that means higher stress to the mechanical parts, that means heavier parts. This limits the maximum RPM which in turn limits the maximum power as opposed to Spark Ignition engines.

The question should be more appropriate, under what operating conditions this question is true. It is obvious that the petrol engines are more efficient than diesel engines for the same compression ratio. Since diesel engines are operated at higher compression ratio, mean effective pressure is more and more power. Generally diesel engines are run at lower speed due to knocking problems. In order to compare any engines one should have some common base line data otherwise it is not appropriate.

I agree that the answer must be more specific, because you can only compare under some conditions, e.g. the same displaced volume. In this case, as has been said, the main reason is that spark ignition engines operate near stoichiometric fuel-air mixtures, while compression ignition engines operate with poor fuel mixtures. Therefore more fuel can be burned per unit time in SI engines of equal displaced volume at a given engine speed.

When people talking about power, which normally means maximum power. The comparison between diesel engine and gasoline engine is a complex question, so I would like to talk some main points:

1. Basically diesel engine has higher thermodynamic efficiency that gasoline engine. Two main reasons for this result: 1) Diesel engine has higer compression engine, due to no "knocking" in diesel engine, as diesel engine has different way to start the combustion. When diesel engine inject fuel too early to prepare too much pre-mixed charge, its work will become very rough, but it is not "knocking"; 2) Diesel engine has no or very low pumping loss, while gasoline engine has very high due to the exist of throttle body.

2. Normally diesel engine has higher torque level than gasoline engine. This is mainly decided by different air intake requirement. Diesel engine is willing to take in as much fresh air as possible, while gasoline have to limit air quantity. Diesel engine has higher torque level also because it has high efficiency and can bear high load level due to no "knocking".

3. So the final point - why gasoline engine has higher maximum power. Power is proportional to "torque * engine speed". So for a vehicle engine, maximum power normally appear at a speed close to its highest speed. For example, an engine has highest speed of 6500rpm, then its maximum power is probable to occur at 5500rpm. As gasoline engine normally has higher maximum speed than diesel engine, its maximum power is consequently higher that a diesel engine with similar displacement.

You may ask then why diesel engine runs with lower speed? Haha, it is another question among may questions. Let stop here.

The time for complete mixture of the fuel and air in the diesel engines is less then petrol one(because of high pressure). So it is necessary to lower the fuel/air ratio and allocate more combustion volume for these engines. So diesel engine gives less power than petrol engine.( Another reason is that because of mixture limited time the rpm of diesel engine is lower than petrol one.) For solving this problem we can use supercharger or turbocharger.

Lets make it simple. One has to define power first. There is horsepower and there is torque. A formula one racing engine has extreme horsepower, high revolution engine, and barley any torque. A diesel engine is opposite. Low revolution, moderate horsepower, and high torque. A VW turbo diesel Jetta is inexpensive and has only 140 horsepower but has 245 ft lbs of torque. A tractor of sorts. It can tow 2000 lbs with ease...try that with any other car capable of 35 city and 48 highway mpg. The efficiency is due to the fact the rpm is set low at cruising speeds and at the high point of the car's torque curve. The car is hardly working....hence efficient. You will be seeing many more diesels in the future because of this, The new 2014 Audi Q5 turbo-diesel SUV is capable of 25 mpg city and 35 highway because it has 450 ft lbs of torque and can tow up to 5000 lbs. Many V8 full size pickups do not approach that kind of power (torque).

Thanks a lot to everyone for contributing answers to my question.

Compression ratio of petrol engine is less than diesel engine. Hence diesel engines have higher brake thermal efficiency than petrol engine. But power point of view heating value of fuel is main factor to have comparison between fuels

In fact a diesel engine is more efficient than a spark-ignited petrol engine having the same power output considering that the engines are installed in a vehicle having a similar mass.

It is simply to consider basic thermodynamics - Ideal Otto cycle (for SI engines) and Diesel cycle (for CI engines). At the same compression ratio and under the cold-air standard assumption, thermal efficiency of Otto cycle is always greater than that of Diesel cycle. When the cutoff ratio (the ratio of the cylinder volumes after and before the combustion process) equals to 1, both Otto and Diesel cycles have identical efficiencies. In gasoline engines, a mixture of fuel and air is compressed during the compression stroke and the compression ratios are limited by the auto-ignition (fuel property) or engine knock. For diesel engines, only air is compressed during the compression stroke eliminating the fuel's auto-ignition or pre-ignition. Then fuel is sprayed into the combustion chamber where it vaporized, atomized and mixed with high pressure and high temperature air, the combustion process takes place. Diesel engines can therefore operate at higher compression ratio and using heavier fuel.

I don't think lower heating value being somewhat lower for diesel vs. gasoline is much of a factor. For example, I have tested two diesel fuels where one has a 10% lower LHV (e.g., biodiesel vs. petroleum diesel) and I adjusted engine fuel management to a higher fueling rate to compensate for the lower-calorific-value fuel in order to achieve the same brake-power-output. The low LHV fuel had a burn rate that allowed this sort of compensation. Even operating a diesel and gasoline engine on identical LHV fuels would result in somewhat higher power for gasoline relative to diesel at comparable BMEP due to the higher rated speed for most SI gasoline engines. Another point of comparison is that diesels are often smoke-limited (or end-of-burn limited) to a fuel-equivalence ratio of about Φ < 0.6-0.7 while SI typically operates at Φ =1 or, when factoring in component protection at high loads, operate with slightly excess fuel.

I think any engine, petrol or diesel one, can make more power. One has to design it like that. Generally diesel cycle is constant pressure combustion cycle. Also diesel combustion is heterogeneous combustion. The diesel spray is atomised at very high injection pressures. Any way it takes time for a diesel spray to burn. So a diesel engine needs more stroke i.e. more piston distance travel. Also to achieve more pressure, it needs to be strong and hence bulkier. All this results slower running engine as compared to petrol one. But it does not mean less power. But more stroke and higher pressures produce more torque, almost double the available power and for higher capacity engines even more than that.

So the application of diesel engines is for heavy duty applications which inherently

need more torque at lower rpm. Examples include Ship propulsion, trucks and buses, tractors etc. Modern diesel engines for passenger cars tend to be more closer to petrol engines through computer control, better injectors and pumps.

But any way, for same capacity or displacement of the cylinder, the diesel engine generally produces less power. Said another way, for same power, a diesel engine has to be bigger in size. This is because of heterogeneous combustion. There should be more air available inside the cylinder than the stoichiometric quantity so that the diesel fuel can burn efficiently. One way to increase power is to use a turbocharger and change the settings of the injector. But this also increases the thermal loading on the engine. This also governs the design of combustion chamber and particularly the piston crown shape. It should be such that it produces high turbulent swirls that enhances mixing of fuel and air.

In contrast to this, petrol engines are based on constant volume heat addition. The homogeneous petrol air mixture burns very fast once ignited. Also the pressure reached are not very high. Also this makes a petrol engine run a higher rpm, sometimes at 20,000 rpm for super cars and super bikes with special fuels. But then the torque produced is lower than that from a diesel engine. So it is suitable for passenger cars which needs to go at high speeds but are light weight, so do not need more torque for accleration.

As an example, a typical bus in India has 150 Bhp with a diesel engine but a torque of 400 Nm and a sedan also has around same power but much less torque with petrol engine. But see the load being carried in the these cases.

I hope this will provide some light on this topic. Thanks.

Because of higher calorific Value of the petrol, thermal efficiency is higher.

Comparison criteria are more significant when they are based on dimensionless variables. For example, an appropriate comparison criterion may be the efficiency of an engine type based on the compression ratio, keeping constant other dimensionless relationships between operating parameters.

With respect to the power, a comparison criterion may be the ratio between BMEP and a pressure characteristic of the cycle. This ratio represents power normalized with respect to displacement, rotation frequency and pressure. The maximum pressure of the cycle determines material thickness and weight of the engine. If this pressure is selected as the reference variable, the above ratio gives an idea of ​​the power produced relative to the size, weight and speed, with interesting technological significance.

Without comparison criteria, this question does not have any significance

The diesel depends on compression ignition. This can only be achieved if the temperature of the air at the end of the compression stroke high - higher than in a petrol engine. Following fuel burn the peak temperature of the cylinder contents is higher in a diesel than in a petrol engine. It is this higher peak (absolute) temperature that results in the diesel having a higher thermal efficiency (check the second law of thermodynamics) than the petrol engine, which shows up in a road vehicle as more miles per gallon of fuel.

The calorific value of diesel fuel is higher than petrol but only by about 9%. The mpg is higher by about 40% and can only be explained by the higher Carnot efficiency as I explained earlier. However, the kinetics of the energy conversion (rate, power) depend on technical characteristics of petrol and diesel engines. Maximum speed and acceleration are likely to be different.

With respect to the power, a comparison can be made by the ratio between BMEP and a pressure characteristic of the cycle. This ratio represents power normalized with respect to displacement, rotation frequency and pressure. The maximum pressure of the cycle determines material thickness and weight of the engine. If this pressure is selected as the reference variable, the above ratio gives an idea of ​​the power produced relative to the size, weight and speed, with interesting experimentall significance

The ideal Diesel and Otto thermodynamic cycles show very little resemblance with the thermodynamics of modern internal combustion engines, be it spark ignited or compression ignited. The difference, as some people have already indicated, has to do with the heterogeneous vs. homogeneous fuel distributions in Diesel and SI engines respectively. A Diesel engine with direct injection of the fuel requires a factor 1.3 or so excess air in order to avoid excessive soot emission. This results in a factor 1/1.3 less fuel for the Diesel engine assuming the same amount of air. Another limiting factor for the Diesel engine is engine speed. A Diesel engine has to withstand higher cylinder pressure due to the initial premixed combustion which means heavier pistons, connecting rods and crank shafts. This means that the engine speed has to be limited which will also limit the power.

I think the best short answer is revs. The diesel engine requires much higher compression ratio than the otto cycle engine, meaning heavier pistons, rods, etc., and it's a constant pressure cycle (slow burn), so diesels don't rev as fast as otto cycle engines? Since power = torque * revs * a constant, one has to balance the potentially higher torque of the diesel against the lower rev limit, for any given engine displacement. Otto cycle engines typically end up with a higher power rating.

Of course, any sort of turbo or supercharging effectively changes the displacement, compared to naturally aspirated engines, so that invalidates any comparisons made purely on equal displacement of the engines.

The power is the torque times the rpm. Diesel engines are limitred in the max RPM allowed, to something like 5000 rpm. Gas engines can run up to 20,000rpm in racing, but for normal use, limited to 7000 rpm. The efficiency of the diesel is higner because of the high compression ratio; 14..19 to 1 while gas comression ratos are limited to 10 to 1. At pert load, the the gas engine is trottled, which causes pumping looses, whereas the diesel is not trhottled so there are no pumping losses.

As the compression ratio in diesel engine is higher than the Gasoline the thermal efficiency in case of gasoline engine would be higher as there are more air molecules per fuel molecule resulting in better burning and higher output. Hence better efficiency.

I suppose it depends how one defines efficiency. Fuel economy, yes, but let's say, horsepower per unit mass of the engine? No.

Now the discussion seems to have shifted to efficiency rather than the original question of, say, power density.  Starting with some practical numbers on efficiency in terms of consumed fuel in mass per unit power per unit time, 2011 Ford presentation says that they have achieved (or at least targeted) 245 g/kWh at BMEP=20 bar and 2000 rpm on In-line 4 engine (it does not give specific descriptions on the I4 engine, I would assume it might be around 2.0 to 2.5 liter4-stroke engine.  The presentation material is "Advanced Gasolene Turbocharged Direct Injection (GTDI) Engine Development, Corey E. Weaver, Ford Research and Advanced Engineering, May 13, 2011.).  On the other hand 235 g/kWh of fuel consumption is not even a state-of-the-art number for similar size diesel engines for a similar operation condition.  There was a discussion on the fuel difference, and there is no unique gasolene/petrol nor diesel fuel, but in general gasolene has higher lower calorific value than diesel in mass basis, one report saying that gasolene's lower calorific value is 43.8 kJ/g while diesel is 42.5 kJ/g, a factor of 1.03 difference.  Translating this to the fuel consumption numbers I cited, 245g/kWh for gasolene engine versus 228 g/kWh for diesel engine.  At least in this comparison diesel engines are 7% more efficient than gasolene engines for the same energy input.  Good things for diesel engines are 1) higher compression ratio so more work is done, 2) in general conversion to pressure (linear movement of gas molcules) is better in diesel because of higher air fuel ratio, 3) lower heat loss to the power cylinder again due to higher air fuel ratio (, which means lower average temperature during the expansion stroke) and better utilization of fuel (lower emission of unburnt hydrocarbons and CO).  The bad things of diesel engines are 1)higher mechanical friction and 2) potentially higher blow-by gas.  In the real world there is no Otto cycle nor Diesel cycle.  If you measure cylinder pressure as a function of crank angle, it will be readily confirmed.  Whether it is spark ignited or compression ignited, once flame kernel or kernels are formed the rest is governed by propagation of flame in the fuel air mixture.  The propagation of the flame front is more of a function of air or gas motion (turbulence) inside the cylinders than the speed of chemical reaction, turbulence being not much different between gasolene and diesel.  In the old days cylinder pressure traces of gasolne engines fluctuated cycle to cycle much more than those of diesel.  All these define heat release rate then pressure as a function of crank angle, which ends up with indicated work done by gas.  Therefore, Otto vs Diesel cycle comparison is not a useful one to explain the diffrence in the thermal efficiency of both types of engines.  By the way since at least at higher than middle torque more than 99.9 % of provided fuel burns completely in recent direct injection diesel engines, discussions on potential poor combustion does not lead to anywhere when comparing efficiency.  This also can be readily confirmed by measuring exhaust gas emissions.

I do not think anything else can be added to the answers. So. I'll try to condense and clarify the answer.

The work produced per unit time by a thermal machine is given by:

• w = thermal efficiency * del of specific enthalpy * mass flow rate involved.

No matter how do you do that.

Corollary:

The Diesel cycle is more efficient (incidentally) because it have a greater expansion ratio (the most important point) and with it a lesser exhaust temperature, and because it does the heat input at an higher initial gas temperature. His specific power, for a fixed geometric displacement, is lesser than of a petrol engine only because it moves (usually) less working fluid (including fuel).

I apologize for my English and I hope it has not hindered the understanding of the subject.

Lisandro,

I understand both your condensed summary and your ethical dilemma. People such as you are getting scarcer these days. Idealistic youngsters who begin their careers by bristling at the wrongs perpetrated by the powerful, finally succumb because they are unable to fight it. This is a sad state. I hope that you will succeed in your struggle. May your convictions live long!

Haha, some crank downvoted my personal response of encouragement to Lisandro. Not that it bothers me. It is good to know that there is one reader in RG who understands simple English for whom the more complicated technical explanations to questions are too much. Goes to show that it takes all kinds to make the world.

BMEP is a good index as to understand engine capacity to develop torque. Disregarding units, power is the product torque times RPM. Fundamentally spark ignition engines develop more power because they can run faster. There are several reasons why Diesels cannot work properly above, let's say, 4000 RPM: the sudden heat release during premixed combustion and the high compression ratio are the main factors.

A conventional gasoline engine burns an air fuel mixture with a mixing ratio of one (as the catalyst in the exhaust pipe requires a stoichiometric combustion gas) . The mixing ratio in a diesel engine is always lean.  Thus, there is excess air which will not be burned, but requires volume in the cylinder.

The chemical composition of gasoline and diesel fuel is different: Dieserl oil is a little denser and has longer hydro carbon chains. But the effect to the energy conversion is weak. The energy content (lower heating value) of Diesel is raughly 42.5 MJ/kg, that of gasoline 42.7 and 43.5 MJ/kg, depending on the type. As the desity of is some 5 to 10 % higher than the desnity of Gasoline, Diesel oil comes with a little higher energy density per volume.

But you should consider what you mean with power density. It is good custom to express the power density of an internal combustion either in kW power per Liter displacement or, more scientific, as mean effective pressure (which is is a kind of specific energy; refer e.g. to John Heywood: Internal Combustion Engine Fundamentals,  McGraw-Hill, New York, 1988, ISBN 0-07-028637-X)  As modern diesel engines are always supercharged, the energy density of these engines rises subsequently with the air density in the supercharged intake. And today, diesel engines have much higher power density than gasoline engines which usually are not supercharges (knocking).

For illustration, I enclose a chart which shows the performance map of a state of the art, naturally aspirated and stratified charged gasoline engine on the left and a highly supercharged diesel engine with direct injection on the right.

Hi

I think a more pertinent question today may be that given an equal task, say propelling a car for 10 miles at conventional speeds say 50 mph, what emissions would each car (petrol or diesel) produce.

Including the difering types of emmissions produced and thier detrimental effect on people/the environment.

The efficency of the engine,the cost of fuel, etc may now secondary to this.

Once you answer this question you need to ask which fuel produces which emissions during its production to get a true picture. 

Kenny

Dr. Brown,

The answer depends far more upon regulations within a particular jurisdiction than choice of combustion system.  Technology exists and is in mass production for near-parity of criteria air pollutants (e.g., NOx, NMOG, PM) and a number of air toxic pollutants between SI-petrol and CI-diesel engine applications.  Reduction of pumping losses and other efficiency improvements with SI engines now entering the marketplace has resulted in efficiecy that approaches (though not yet equals) diesel efficiency over typical drive cycles and the lower carbon intensity of gasoline relative to distilate diesel fuels will result in roughly comparable CO2 emissions for both types of combustion as SI engines continue to improve.  North America has had fuel and comubustion-system neutral emissions standards for both heavy-duty and light-duty sectors for approximatley 10 years. 

The most incredible engines of today are F1 engines (after 2014), reaching near 50% efficiency. They are fueled by petrol (gasoline) but it seems that the way they burn the fuel is not typical of Spark Ignition, but of Compression Ignition by the process HCCI. If that is the case (almost nobody knows this well kept secret) I think the type of fuel they use if a key factor for these engines. And HCCI engines are not limited by octane rating of the fuel but, I presume, a mixture of octane rating and cetane rating. However, the first sentence of the F1 regulation for fuel says "The purpose of this Article is to ensure that the fuel used in Formula One is petrol", but there is no minimum values for Octane Number (only maximum) nor minimum values for components such as aromatics. Is it petrol that they use in F1? 

Thank you Jorge. I have always wondered about this question! Perhaps they mix rocket fuel with it !  :-)

Jorge - There is certainly previous evidence that F1 fuels only remotely resemble what we think of as petrol or gasoline. Honda published a paper on their 1.5 Turbo F1 engine in 1989, shortly after the formula changed and turbos were banned. As I recall,  the fuel was roughly half  toluene and required preheat through a coolant heat exchanger in order to vaporize. They were able to push over 5 bar of intake manifold pressure without destroying the engine from knock. I put a link to their SAE paper on the topic in case some of my recollections from reading this 25 years ago are a bit off. Interesting topic, but probably in need of a separate thread of its own. 

I also would be surprised if F1 were using HCCI but I suppose it is possible. HCCI is very difficult to do much above 4-6 bar BMEP and developmental automotive HCCI engines typically transition to a different combustion mode at higher BMEP. F1 has very little part load operation - the throttle is almost digital between WOT and closed except in the pit lane or behind the safety car. Considering the pace of engine development in F1 racing, it really does deserve its own separate discussion thread. 

Yes, I remember the 1.5 Turbo F1, I was journalist for a motoring magazine back then. The fuels at that time did not resembled petrol at all, people had to use masks. But later the FIA imposed near pump petrol and all that changed. 

Yes I agree with you that the conventional way of using HCCI is at part load, but remember that they are using lean and extra-lean mixtures and that counts as part load operation (although not throttled). But it is possible to uso other concepts, such as the Mahle chamber jet on the Ferrari engine. Again, nobody knows about it, but it seems possible. This is not HCCI but uses a way of producing a very fast and efficient combustion required for the burn of lean and extra-lean mixtures. Remember the Honda CVCC of the 70s? Same concept, only now with precise control. 

If not for these combustion efficient systems, how can they produce near 50% efficiency from racing engines? The calculations are simple: over 700 bhp from 100 kg/h of fuel. And we are talking about the engine, not the overall powertrain. 

Shall we start a discussion on F1 efficiency? I will do it.

Power is a pure mathematical concept in an engine. The engine output is torque.

if you want a simple answer diesel engine due to heavy moving parts cannot operate at a high RPM but petrol engines can easily go up to high RPM since it has less moving parts.

So power which = 2*pi*RPM*Torque

So due to higher RPM operation petrol engines have a higher power even though they might have relative less torque compared to diesel engines. since Power is directly proportional to the Engine Speed.

The calorific value of diesel fuel is roughly 45.5 MJ/kg (mega joules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/litre compared to 33.7 MJ/litre). As the Calorific value of the Petrol is more than the Diesel, we get more power.

You have to refine your question: are you talking of engines of the same capacity? Are talking of engines without turbo-charging? If you are talking of engines with similar capacity and atmospheric, the gasoline produces more power as it will use all the intake air (uses stoichiometric conditions) while the diesel engine can only burn lean mistures (does not use all the air) and the gasoline engine revs at higher speeds. However, today highly turbo-charged diesel engines can reach the same level (or higher) of power as the gasoline engines (231 hp from the 2.0L BMW engine). 

for same compreesion ratio efficiency of otto cycle is more than diesel cycle and for same heat input the heat lost is lesser in otto cycle. for lesser amount of heat loss, there is more heat converted into power in petrol engine