Dear Steffen,

the theory states that higher the compression ratio higher the engine efficiency. However, in real applications this is limited by the escessive combsution noise that can lead to excessive engine wear. In addition, for higher compression ratios, the combustion temperature will increase leading to higher NOx formation by thermal mechanism. The advantages and disvantages will be also related to the type of combustion that you will use. In general, for any combustion, the disavantages are that you are increasing the mechanical demand, forming more NOx and reducing your control on the combustion process whilst yo are increasing the thermodynamic effiency and reducing some pollutants as unburned hydrocarbons.

Best Regards,

Rafael

From one point of view, increment of rc up to 10 in IC engines would decrease the heat transfer rate of the engine at about 10%, so you would have a lower (heat transfer/brake power) which is an significant parameter to keep the performance of an engine in an efficient condition, but if you increase the compression ratio over than 10, the heat transfer rate of the engine would be boosted and the performance of the engine would be decreased. These behaviors are the results of changes in combustion parameters, the parameters such as flame speed and gas movements in cylinders.

I can't understand your answer Sina.

what do you mean by " heat transfer rate "?

Do you mean "heat transfer rate from the walls"?

Coolant heat transfer.

Ok

High ratio increase the thermal efficiency but with limit value to avoid knokonig at combustion

As above, higher compression allows greater efficiency of combustion, but the limit is pre-ignition due to the high presure and consequent heating of the charge. The practical limit for currently available petrol in the UK is about 10.5, but if you want to buy expensive 110 (RON) fuel than about 12 is possible. Methanol fuel will let you go to 14. See: https://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/fuel-octane-power.pdf

As we know diesel engine has more efficiency due to its high CR.

But why MAZDA decreased CR as we at the following link?

http://www.mazda.com/en/innovation/technology/skyactiv/skyactiv-d/

Steffen,

The answer to this lies in the way legislation is moving in Europe and the US.

Nitrogen oxide and Nitrogen dioxide are produced in copious amounts when combustion is at high pressure - 80 to 100 atmospheres which the engine produces as the burn progresses.

NO and NO2 pollution from automobile diesel engines is seen as the prime cause of high early death rates in densely populated areas.

So you can expect to see legislation that severely restricts NOx gas output in the immediate future.

Lowering the combustion pressure significantly reduces production of these gases. So there is obvious commercial advantage in developing a NOx clean engine and grabbing the patents before the competition.

So we can say a trade-off between efficiency and pollution...

I think the answer to that question is 'perhaps'. We have seen the automotive industry rise to a significant number of challenges as legislation has moved over the decade and particularly as oil/fuel prices have risen.

The trade off you suggest is just a challenge to the technologists in that industry. Diesel engines will die a commercial death unless an effective balance between efficiency, pollution, cost of running and a host of other competing aspects is found

So what do you think about Mazda 1.5 gasoline engine with a CR=14 (like diesel...)

If you increase the CR, normally you are able to increase the RON for the engine, then you may change the ignition timing to be closer to the TTC. As result, you will have a less unuseful power, i.e. in the compression stroke. It means that both the engine torque and power will be higher.

Higher CR is related to knocking problems of S I Engines. It may be raised upto higher to get max efficiency.

The advantage of higher compression ratio is reflected best by SKYACTIV-X Mazda Engine. An engine that actually uses knocking mechanism in general to create combustion from very lean operation.

As we know that both SI and Diesel engine are staged combustion based:

A. Initiation

B. Development

C. Propagation

D. Termination

Just like free radical reaction in chemistry, combustion in engine also is dominated by free radical reaction. But, the problem with staged combustion often involves boundary between unburnt and burnt mixture regions, resulting in undesireable emissions. The non-uniformity between unburnt and burnt creates emission for example hot-spots in SI engine create NOx, while realtively lower temperature zone in diesel makes un-burnt carbon emission.

In order to deal with staged combustion, an advanced method of simultaneous combustion due to initiation of combustion at multiple spots, rather than a point, HCCI (Homogenous Charge Compression Ignition ) mode was developed allowing to create premixed lean mixture ignition assisted by higher compression ratio. But, HCCI could never performed for all load ranges of engine performance operation.

Now, SKYACTIV-X is most innovative method of combustion, that utilizes higher compression ratio, to compression air-fuel mixture (pre-mixed) to close to auto-ignition temperature, then utilizing spark to initiate combustion. Alone, compression has unpredictable combustion characteristics like CA10, CA50 and CA90. Spark is used to make combustion predictable, as spark is one that initiates, despite compression work done on mixture. Higher compression ratio generates near HCCI condition, then with spark flame sphere, combustion of air near spark plug results in expansion mixture (burnt mixture), that compresses further surrounding (un-burnt mixture), since we know that combustion creates higher volume products as higher no. of moles of products are created compared to moles of reactant consumed. The expanding, burnt mixture forces rest mixture to combust in HCCI mode, hence better emission and leaner operation.

When a secondary combustion wave is created in SI engine, due to compression from burnt mixture, it is categorized as knocking in SI engine. Here we are utilizing knocking effect to burn leaner mixture with more efficiency.

The problem of combustion based alone on high compression arises due to uncertain heat loss characteristics of cylinder wall, which effects in-cylinder pressure and temperature (that's why HCCI is unsuccessful) .

Here we do need to calculate cylinder pressure and temperature to determine spark timing, but we are not chasing accurate auto-ignition condition, instead we are targeting thermodynamic condition before auto-ignition, which actually takes care of cyclic inaccuracy of calculation, as spark sphere compensates it, with simultaneous and bulk mode of combustion (not staged). Therefore higher stability at larger load range.

Dear Steffen,

An engine limits power by reducing the intake of fuel and air to an engine; if only half the fuel and air is entering a piston, the compression ratio is effectively halved as well. Considering all the advantages of high compression, one might wonder why anyone would not use a high compression ratio.

Benefits

(1) CR influence on maximum torque

(2) High speed. At highest engine speed, the CR reduction is less effective

(3) Smoke becomes the limiting factor. Smoke drop can be exploited to improve the torque. Working only on the SW and Prail reduction, pumping and friction loss can be lowered and intake duct permeability increased. HCs&CO can be also lowered.

(4) Engine speed limit is less critical. Combustion phase optimization and friction losses reduction do not cause the deterioration for low CR engines.

(5) It uses less fuel than a spark-ignition engine.

(6) It uses the fuel more efficiently (in other words, less power is lost to the actual ignition and to excess heat).

(7) Since less fuel is used, the car pollutes less

Drawbacks

(1) With high compression ratio, they operate at higher pressures and offer a cleaner and complete burn of fuel leading to better mileages too. However, it's not ALL good. The higher pressure requires stronger build of the engine (in some cases heavier). Chances of knocking in the engine are higher. All components are subjected to high stresses and if not designed properly may lead to early failure of engine components. Modern day motorcycle engines have extremely good brains behind the design and offer high compression without compromising engine life. Basically, they are mechanical wonders made by great engineers.

(2) What's really important is dynamic compression ratio. This throws the camshaft into the calculation, as compression cannot actually happen until the intake valve has closed. That's why bigger cams suggest a higher static compression ratio. Since the bigger cam closes the intake later, the piston is higher, and the effective or dynamic compression will drop compared to a shorter cam. So use a higher compression ratio to compensate.

(3) With High CR affects the horsepower. An engine that sounds and runs lazy.