Engine Efficiency #2

The next efficiency that has an effect on the way an engine performs is thermal efficiency. As you can imagine this has something to do with heat, that is to say heat not temperature. In case you are not quite clear on this subject, heat is a reference to one of the basic forms of energy, and we typically measure this in Joules or British Thermal Units (BTU). Temperature is only a measure of the intensity of heat energy and we measure this in degrees Celsius or Fahrenheit depending on your unit persuasion.

One joule of heat is equal to the amount of heat given off by the human body at rest over a period of about 15 seconds. The temperature of the human body is 37° C, but if you were to take that 1 Joule and spread it out over the size of a medium sized living room, the intensity of the one Joule of heat would drop significantly so that the temperature of that room would be something very cold.

Gasoline is burned in the engine in a combustion process that converts the chemical energy of the gasoline into very concentrated heat energy. This concentration of heat causes a very rapid expansion of the compressed air in the engine’s cylinders. This expansion of air acts against the piston, pushing it down in the cylinder, which causes the crankshaft to turn. The more accurately the fuel and air are mixed, and the more precise the combustion process, the more efficient the engine will be.

The amount of energy that comes from the fuel that can be turned into force to move the piston down in the cylinder, rather than just turned into heat that will eventually be lost, the more thermally efficient the engine is said to be. Overall, internal combustion engines are very thermally inefficient because most of the energy in any engine is actually turned into heat and lost. Considering that super heating the air to create the expansion necessary to push the pistons, this excessive heat and the loss thereof is not really a surprise, because the heat cannot all be directed toward moving the piston. About half of the lost heat actually goes out the tailpipe, and the other half of the heat goes out through the cooling system and radiator.

The average gasoline engine has a thermal efficiency of only about 20%. This means that only 20% of the heat energy from combustion gets turned into work that moves the piston. This number is pretty low but it is much better than it used to be and will probably get better with time, however, most engineers theorize that that gasoline engine thermal efficiency can probably never get any better than about 35 or 40% due to the constraints of the laws of physics. A diesel engine has greater thermal efficiency than a gasoline engine which is the reason that diesels get better fuel economy. A diesel’s thermal efficiency can be as high as 30% for diesel engines found in cars and trucks and around 40% in many industrial equipment applications. Although this is an improvement over the efficiency of the gasoline engine it is still very low.

Diesel engines have higher thermal efficiency because they can operate with a much higher compression ratio than a gasoline engines. This is because the fuel is injected into a diesel engine just before the piston is at the top of the compression stroke. In most gasoline engines the fuel is injected onto the back of the intake valve and drawn into the cylinder on the intake stroke. The fuel then gets compressed with the air as the piston moves up in the cylinder. As this piston is moving up it is compressing the air which concentrates the heat energy in cylinder. When this heat becomes concentrated under compression, the intensity of the heat goes up. This increased intensity, or temperature, can ignite the fuel before the appropriate time arrives which will cause engine knocking. This is a terrible condition that causes the engine output to go down, and can potentially damage engine internals.

Direct injection has an injector that sprays fuel directly
 into the combustion chamber

Many engine manufacturers are building more and more direct injection gasoline engines. These engines inject the fuel in a very similar many to the way diesels inject fuel. This gives these gas engines greater thermal efficiency because they can run well with higher compression ratios. The compression ratio cannot be raised to high however, because the higher pressure that results causes a loss in the mechanical efficiency of the engine, not to mention the fact that the engine must be built to be more robust in order to withstand the higher pressure.

Next up, volumetric efficiency.