Building on previous work, in 1892 Rudolf Diesel was granted a patent for a variation on the Otto cycle gasoline engine that he felt offered a number of advantages over the currently available gasoline engines of the day. The primary advantage was then and remains today, a more efficient engine getting more effective use out of the energy available in the fuel being burned in the engine.
The changes in that Diesel made come down to two things: First, in a gasoline-powered four-cycle engine the fuel and air are drawn into the cylinder together this allows for simpler engineering, but places an effective limit on how compressed the fuel-air mixture can become before the compressed vapors will self-ignite (Don’t worry, I’ll explain that more a little later on.);
Secondly, ignition of the mixture of fuel and air is done electrically by means of a spark plug. Diesel noted the tendency of high compression to cause self-ignition of the fuel-air mixture in the gasoline engines and decided to build an engine where, instead of being a problem, that would be an advantage.
So, how does this self-ignition occur?
When a gas changes pressure its temperature is affected as well. To understand this we have to look at what temperature is. Temperature measures the average molecular kinetic energy, or to think of it a different way, how often the molecules in the substance bounce off each other. The more molecule on molecule collisions in the substance occurs, the hotter the substance will be.
So, if we have a balloon of air at, say, one-atmosphere pressure, and it is at temperature X, then we shrink the balloon so that it contains half the volume it used to hold, the pressure in the balloon will go up by a factor of 2. And suddenly, because the molecules in the balloon have less room to move around without colliding with other molecules, the temperature of the air in the balloon will go up, as well.
Thus we can see that if you increase the pressure of a sample of any gas, it will also increase the temperature of the gas as well this is why, when pumping up a bicycle tire the pump gets so hot, the air in the pump cylinder is being pressurized, making its temperature rise as well.
Likewise, if you reduce the pressure of that gas sample, the temperature will drop this can be seen when using an aerosol spray can, it’s not just imagination that makes one think that the spray comes out cool. Both of these are called adiabatic temperature changes. That’s a big word that simply means the temperature changed without actually putting any heat energy into the gas, or taking any out.
The first stroke of the four stroke Diesel engine is almost the same as the first stroke in the Otto cycle gasoline engine: It’s an intake stroke, but where a gasoline engine draws in a mix of fuel and air, the Diesel engine draws in just air. The second stroke of the Diesel engine is identical to the gasoline engine the compression stroke. The air in the Diesel engine’s cylinder is being compressed to a much higher pressure than would be happening in a gasoline engine, and there is, as yet, no fuel in the cylinder. Now, as the air is being compressed it is also rising in temperature, through the adiabatic heating I’d mentioned earlier.
When the piston is at the top of its travel in the cylinder, with the compressed air at high pressure and temperature, the fuel injection port opens and shoots in a measured spurt of fuel into the cylinder. When the fuel mixes with the compressed air the high temperature of the compressed air in the cylinder makes the fuel and air combust without any additional energy needed eliminating the need for spark plugs.
(Diesel engines may use glow plugs, but these are simply an additional heating element to help the engine get up to operating temperatures in cold starts, during normal operation, they aren’t needed, or used.) Incidentally, it was the engineering of the injection port that proved to be one of the most difficult parts that Diesel faced in making his engine because it has to force fuel into the cylinder while the cylinder is near its maximum pressure, then it must act as a seal during the combustion of the fuel and air, or the force that Diesel wanted to harness would leak back out through the fuel port.
The third stroke of the Diesel engine is identical to the third stroke in the Otto cycle engine the power stroke, where the energy of the combustion of the fuel is transferred to the crankshaft of the engine. And the fourth stroke, too, is identical to the exhaust stroke, expelling the used combustion products from the cylinder in preparation for the next cycle.