Thermal Efficiency

The Basics of Efficiency

A common method of measuring an engines efficiency is to look at the thermal efficiency. This allows a fundamental understanding of where the energy in the fuel goes during operation. Many people don’t realize that at best, about 1/3 of the energy in a given fuel is actually converted to useful work in an internal combustion engine. That means out of each gallon of gasoline you put in your car, only 1/3 of that gallon is used to power your vehicle and accessories. So where does the rest go? Well, there are two primary places, that is in waste heat and in friction, which really boils back down to another form of waste heat. This is where the term thermal efficiency comes from, as the internal combustion engine is really just a heat engine. In fact, even the power produced by the engine for useful work can also be viewed as a conversion of thermal energy into work.

The thermal efficiency of an engine is a great way of looking at an engine’s ability to convert energy from a fuel into useful work with a minimum of waste heat production. So what factors affect thermal efficiency? For starters, compression ratio is a big one. The higher the compression ratio, the higher the TE. This helps explain the high thermal efficiency of a diesel engine, due to their much higher average compression ratio, typically around 18:1. So why do most vehicles have a compression ratio of approximately 10:1? Primarily, this is due to practical limitations of keeping the fuel from pre-igniting due to excessive heat on the top of the piston and the combustion chamber. With higher compression ratios come higher operating temperatures, and there are practical limitations to how much cooling is possible.

Combustion chamber design, intake manifold design, spark plug position, valve design, piston crown design, and exhaust manifold design all have an influence on an engines TE. It turns out that the amount of fuel you fully combust also has an impact on an engines thermal efficiency. Of course, this makes sense because any unburnt fuel would certainly not be turned into work. But how does the intake manifold affect TE? Well, there are a lot of complex factors that go into how well a particular combustion chamber burns fuel, but the way the intake charge (air and fuel) enters the combustion chamber and mixes affects the completeness of the combustion. If you look back to the VE article, you will recall that air, as a fluid, has momentum and will try and maintain that momentum. If the entire intake path is designed such that the air is swirling on its way into the combustion chamber, this will cause improved mixture of the air and fuel and promote a more complete burn.

The combustion chamber can have a similar effect on the combustion process. As the piston is moving toward the combustion chamber, the air fuel mixture is squished. How the intake charge is squished depends on the shape and design of the combustion chamber. Since there are practical limitations to the position and the number of spark plugs available to ignite the intake charge, the combustion chamber and piston crown design have a crucial role in promoting complete combustion and squeezing any pockets that might be far away from the spark plug.

But how can an exhaust manifold make any difference? Well, as was discussed in the P-V Cycle article, the exhaust manifold design promotes scavenging of the burnt gases, and a well designed exhaust will effectively vacuum the burnt gases out of the chamber, which in turn makes the intake charge rush in that much faster. This added turbulence can improve the completeness of the combustion of the air fuel mixture. The significance of all these factors is heavily influenced by the director for all this air coming in and going out, the camshaft. What helps an engine run at high efficiency at low rpm is different than what helps an engine run at high efficiency at high rpm. Here we go with the compromises once again.