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How does a diesel engine work?
Everybody has a few ideas about how diesel engines work, but in reality, very few people know exactly what the principles behind them are, or rather, what the major differences are between the offspring of Rudolf (Diesel) and that of Nikolaus-August (Otto), the father of the gasoline engine. So here is a brief snapshot of the similarities, which are many, and the differences between the two.
First, you need to know that the two types of engines use all the same components (pistons, valves, camshafts), that the cycles are identical (intake, compression, combustion, exhaust) and that the intake, supercharging and exhaust systems are very similar. Nevertheless, there are some fundamental differences, especially related to the intake and combustion of the air-fuel mixture.
First of all, the diesel engine draws only air into its cylinders, and the fuel is injected only at the precise moment that is optimal for producing power. The conventional gasoline engine with indirect injection, always draws in an air-fuel mixture more or less perfectly put together, meaning that the two were mixed before the intake valve is closed.
Next, in the case of the diesel engine, it is the injection of the fuel (under high pressure and finely atomised) into the highly compressed air in the cylinder that produces the combustion reaction; this is the pre-ignition. The gasoline engine, for its part, needs an ignition device to trigger the combustion of the compressed air-fuel mixture, which is the role of the coil and sparkplug.
There is also a difference in engine control, which stems from the fundamental principle of each. In a gasoline engine, the accelerator generally activates a valve, the throttle valve, which allows the engine to receive more or less air in which the injection system will have mixed the gasoline in very precise proportions. The gasoline engine is therefore never satisfied, since it is deprived of part of its air semi-permanently, except obviously when the pedal is to the metal.
In the case of the diesel engine, there is no throttle valve, so it draws in all the air that it wants, from which comes its hoarse intake sound. The accelerator directly commands the fuel injection mechanism and module the quantity of fuel, according to its needs. It functions therefore in excess of air, especially at slow speeds, which explains why diesels produce so little heat at idle.
So a diesel engine is only a four-stroke internal combustion engine whose power phase is sparked by a different kind of combustion.
Now, this Diesel cycle, named after its inventor, needs special components and different engine management systems than those of the Otto cycle, which means that aside from some nuts and bolts, the two share nothing.
If I was not clear enough, please do not hesitate to pose questions, no matter how simple they may be.
6 Comments
Comments
Ron, modern diesel engines now can start easily at sub-zero temperatures because the starting process is very different than before.
In the old days, heating devices (the glowplugs) in each cylinder were needed to raise the temperature of a cold engine so it would start.
Now, with the advent of modern electronic injectors, the ECU (the engine’s brain) is capable of retarding the moment of the fuel injection in the cylinder to let the heat generated by the compression of air reach the critical temperature. There’s still glowplugs, but they are not needed for starting when temperatures are above -24C.
Now, your question made me wonder if Diesel engines are popular in cold countries like Sweden. According to this Swede study, http://www.naturvardsverket.se/upload/english/06_climate_change/pdf/summary_emissions.pdf, 34 % of new car registrations in 2007 were diesel-engined. They must know something we don’t about cold starting.
I forgot to specify that the Swede study above is from Sweden’s Environmental Protection Agency. http://www.naturvardsverket.se/en/In-English/Menu/
thanks for those great information but let me ask about what makes the air compressed in the inaitial start of the diesel engine (plase i need to know)and is the battaries have any thig to do with this procedure
Mohamed,
The air compression in the cylinder comes from the rotation of the engine, which is produced initially by the electric starter. So, in a way, yes the car 12V battery do start the process. The same can be said of the gas engine, too.
Funny fact : giant Diesel engines (mine trucks, as an example) don’t always use electric starters. Some use compressed air (produced by an engine-driven compressor and stocked in an air tank) to turn the engine.
But the strangest starter I know of is a “starting gas engine” for the giant diesels ; in very, very cold temperatures, in the old days, the operator would start a normal gas engine, let it heat a bit, then he would engages a transmission that would turn the big diesel engine fast enough to make it start. But that was in the old days, before the TDI…
Question: What components would require replacement if gasoline is accidentally ran through the clean diesel system? For example all lines, pump, injectors but what about the particulate filter? Catalytic converter?
September 29, 2008 by Ron Arbour
This new clean diesel seems to be a very good engine from what I can see. I have been considering purchasing a Jetta Wagon but what concerns me is cold weather starting. In the past I knew several people who had the old technology and they had to plug the vehicle in when we with gasoline engines didn’t bother.
At work there were no electrical outlets so one person I worked with would leave his vehicle run all day when the temperature plunged.