The Diesel Engine


Please refer to my previous article here about the 4-stroke piston gasoline engine as this article will require that knowledge.

     For the third installment, I’ll be describing the diesel engine, and how it’s unlike its petrol brother.  The design of this engine isn’t as unusual as the rotary, but there are a few significant differences in the way it works.

     For starters, a diesel engine uses the conventional 4 stroke piston design.  No weird triangular rotors here.  But there are two big changes from a petrol engine:

     1) There are no spark plugs.

     2) The use of diesel fuel (obviously)

     Say what?  Don’t we need spark plugs to burn the fuel and create the combustion in the cylinder?  With no combustion, we can’t harness the latent energy from the fuel?  Well this is where a diesel engine differs.  Instead of using a spark plug to ignite the air/fuel mixture, we use pure compression from the compression stroke to heat the air, and then inject the fuel.  When we inject the fuel into the highly compressed air, that automatically self combusts (due to the already high pressure and temperatures), resulting in our release of energy.  Remember, the main objective of an engine is to harness the latent energy from the fuel and convert it into kinetic energy which can turn the wheels and move the car.  Diesel engines just use a slightly different way to achieve our target.  Here is an animation showing how it works (click the image if it doesn’t animate):


     It looks exactly like a normal 4 stroke petrol engine, except for the compression stroke.  Instead of a spark plug, we use direct injection of the fuel into the cylinder (instead of most petrol engines which mix the air and fuel before they enter the cylinder) to create our combustion.  The injector is the most complex part of the diesel engine, since it has to both distribute the fine mist all around evenly and withstand the high compression pressure and temperature.  

     And remember that diesel engiens use a different type of fuel; not petrol but obviously diesel.  When crude oil is refined, we get both petrol and diesel fuels (among other stuff) but diesel fuel requires less refining resulting its lower cost (relatively).  The difference is in the energy density of the fuels.  The energy density is essentially how much energy the fuel has.  For petrol engines, a gallon of petrol has about 132 x 10^6 joules of energy.  But diesel has 155 x 10^6 joules of energy.  That’s about 17% more energy per gallon.  And that’s why diesel engines are more efficient.  We get more power per gallon with diesel engines than we do with petrols.  Also, in terms of emissions, diesels release less harmful stuff.  You know, all those global warming stuff.

     Let’s compare diesels vs petrols since if we’ve said that diesels are more effeicient and healthy for the environment, why aren’t there more of them around?  Well for one, diesel engines usually weigh more due to their higher compression ratios, resulting in much more beefy internals (internals being the inside parts of the engine which are stressed due to the combustion cycles).  Also, due to their weight and high compression ratios, diesel engines have a lower maximum rpm resulting in less horsepower but more torquey than their petrol counterparts.  Why?  Because at high rpms, the stress on the interal parts becomes exponential.  With such heavy parts and high temperatures, it would be harmful to the engine.  Diesel engines are also more expensive to manufacture, and the diesel oil is also less readily available despite the fact that they’re easier to refine.

     What the consumer usually sees in a petrol vs diesel comparison is the fuel efficiency and the torque/horsepower rating, meaning diesels have a better fuel efficieny and higher torque but lower horsepower.  And that results in worse acceleration times in terms of performance.  But technology is advancing diesels nowadays to make them more consumer friendly.  Just recently the U.S. started to get diesel varients of the more famous makes and models while Europe has been “dieseling” it up for a while now.  It has nothing to do with diesels being “worse” than petrols, but its the fact that most people associate diesels with huge tractor trailers puffing out huge amounts of black smoke and making obscene noises.  Hopefully that will change in the future, as fuel economy becomes more of a deciding factor in auto purchasing decisions with the price of oil and all that and the proliferation of hybrids forcing consumers to look at engines other than just petrols.


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April 2009
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© 2009 Rusi Li

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