Hello Vishal,

Diesel engines usually produce more sound due to the blowdown event, which is the moment when the exhaust valve opens. Because the internal pressure inside the cylinder chamber is usually higher in diesel than in petrol engines, the sound is consequently higher during blowdown.

Contrarily to the gasoline engine which needs a spark-ignition system for the combustion process of the air-fuel mixture, in the diesel engine the combustion does need a very high compression in order to reach the auto-ignition temperature/pressure of the air-fuel mixture, therefore this high pressure requires stronger parts and does make the diesel engine more noisy than the gasoline one.

As previous answers describe it, there is various noise sources in an engine. But the major one is combustion noise which is a major deal for engine calibration.

Indeed, the engine noise is directly related to the pressure variation created by the combustion in the chamber. By the combustion mode used, diesel produces a higher variation of cylinder pressure which can be heard as the engine noise.

Today, the main solution to this problem is to use a well tuned injection strategy, especially multiple injection strategies. In particular, pilot injections occurring before the main injection as it permits to gradually increase the chamber pressure.

I just though about a reference that you can read if you want more details on internal combustion engine:

http://books.google.ca/books/about/Internal_Combustion_Engine_Handbook.html?id=ESHYjZ0CxwUC&redir_esc=y

There are two main reasons:

first is the premixed combustion in diesel engine, the dP/dt is so large

second is the high compression ratio in diesel engine, the combustion pressure in cylinder is higher than that of gasoline engine, when exhaust valve opens, the burned gas move out with high velocity the long wave, it's make noise

Hello Vishal,

Maybe it could be interesting for you to visit this web site from the French carmaker Peugeot Citroen : http://www.psa-peugeot-citroen.com/en/diesel

PSA is a leader in diesel engine and it tries to explain why diesel engines are not so bad... of course, it's communication from a firm, so you need to be cautious about the information but there is some interesting information. Moreoever you can tchat with enginners from PSA.

vincent

For an empirical perspective on why the noise of the combustion event you could consider the timing events of the diesel spray versus the spark ignition and deduce why the combustion noise of the diesel engine is so much higher.

Diesel main injections can happen in the window of about -20 ATDC up to 5 ATDC and are usually very short within that window. The early speeds of the reactions are roughly relative to the fuel mass flow into the cylinder and industry trends such as multiple injections and ramp rate shaping indicate that fuel mass flow rate is a critical parameter for diesel combustion control.

By contrast, the spark event can occur as early as -40 ATDC up to 0 TDC and although there are a couple modes of spark ignition combustion that occur throughout the burn, they are on average much slower. This indicates that the pressure rise rate should also be somewhat lower.

Since the peak dP/dt of the typical spark ignition engine should be lower than the diesel engine, the noise associated with combustion should also be lower. However, if you have spark induced knocking at WOT you could observe a noise level similar to that of a diesel.

You should only compare the combustion noise specifically. All the other noise effects from fuel pumps, valves, exhaust manifolds, turbochargers, and emissions equipment are not really a fair comparison. Check out Mazda's newer engine block which is manufactured for both types of combustion.

During the suction stroke only air is entering inside the combustion chamber and during compression stroke fuel is injected, so there is no proper mixing of air hence instant burning takes place and increase in cylinder pressure. It works on the Diesel cycle and petrol engine works on the Otto cycle.

With the evolution of direct injected gasoline and premixed combustion / pilot injections for compression ignition, I am going to disagree with the prior statement. Modern gasoline powered spark ignition engine may inject on the compression stroke and many current diesels can run in modes that spray during the intake stroke.

This supports a prior claim I would have made which is also contrary to the prior post; namely that the Diesel cycle is best simple thermodynamic analysis for compression ignition engines while the Otto cycle is the best for spark ignition engines. It is not due to this argument that I hold this position, but it is a requirement for considering the types of combustion modes that have been identified in modern engines, relating to the comments above.

Both of these models individually are really insufficient to describe typical combustion engine conditions while the dual cycle, or combined Otto-Diesel cycle, is much more representative of actual engines in general. In fact, it can work for both the CI and SI engines better than the individual cycles while newer combustion modes are represented more fairly. Of course, if it is an academic pursuit for introduction, it is fine to model the historic engines this way, but only as a stepping stone to more complex cycle analysis.

While I admit there is an added complexity of dividing the heat release event there is a metric, sometimes called the premixed combustion fraction, which could be helpful. For academic problems realistic numbers should be presented for both types of engines as well as a modern engine with an alternative combustion mode like PPCCI or HCCI, or maybe a lean burning GDI with compression of 14.5:1.

Advanced engines are at the point where looking at them as a gas or diesel fuel burning engine is no longer sufficient to categorize them. For now, Spark Ignition, Compression Ignition, and Spark-assisted Compression Ignition are the only real divisions that can be used to divide engines in the manner on topic. All types of ignition systems are now independent of the fuel delivery location and decoupled from the fuel timing. Thus, for all types of engines, fuel delivery is open from intake to (almost) TDC or later.

The fuel we use is only good for knowing which station you need to go to, and even then, there's not one right answer.

1. Higher pressure

2. Higher pressure leads to bulkier vessel - means more noise density

3. Higher valve closing/opening speed - more noise

Due to compression ratio and non homogeneous fuel

Thanks a lot everyone for your valuable answers.

It is because of high pressure performance of Diesel engines and auto ignition characteristics of diesel fuel in high incylinder pressure.

CI perates at higher CR and high pressures

it is because there are three phases to the combustion in a diesel engine

1- ignition delay

2- rapid combustion

3- controlled combustion

so in the second phase when the fuel starts to burn,creating a sudden rise  in pressure that causes the characteristic diesel engine knock.