Thermal Efficiency

[Evil Result]

I'v been searching around for a bit and have had no luck in finding the numbers on Thermal efficiency of modern gasoline engines. I keep reading 20 - 25%, seems a bit under rated for 2004.

If those numbers are higher than stated how much improvement could be gained from using Ceramic anti-friction and thermal barrier coating to increase how much heat energy is transformed into mechanical energy?

How high can the thermal efficiency of a IC engina go with current ceramic coating or something else, 30 - 40%?

[Neutrino]

Originally Posted by Evil Result

I'v been searching around for a bit and have had no luck in finding the numbers on Thermal efficiency of modern gasoline engines. I keep reading 20 - 25%, seems a bit under rated for 2004.

If those numbers are higher than stated how much improvement could be gained from using Ceramic anti-friction and thermal barrier coating to increase how much heat energy is transformed into mechanical energy?

How high can the thermal efficiency of a IC engina go with current ceramic coating or something else, 30 - 40%?

I don't think that will make that much difference. I believe most of the thermal energy wasted goes out trough the exaust and not trough heat dissipation trough the engine.

So the best way so far to recoup those losses has been with a turbo.

[MagicRat]

Coatings can only do so much to improve thermo-dynamic efficiency. At a certain point, the fuel/air charge will become too hot and detonate, instead of burn. Detonation is less efficient combustion for gasoline, and you lose power.

You can easily improve thermo dynamic efficiency by increasing the compression ratio. But you run into the same problem as stated above, by overheating the fuel/air charge.

You can use a different charge, more resistant to detonation, such as diesel. This is why diesels can run at ratios of up to 22:1. Therefore, diesels have the superior thermo dynamic cycle, comapred to gasoline engines.

Other things that can be done is to increase the size of the cylinder and reduce their numbers, although this produces other undesireable characteristic, such as vibration. This reduces the ratio of combustion chamber area to fuel burned, thus improving efficiency. This is why rotary engines have inherently lower thermal efficiency, and subsequently less fuel economy, in part because of the amount of combustion chamger surface exposed to the burning fuel.

Finally, thermal efficiency can always be improved by adding a turbo. It recaptures some of the thermal energy wasted as expanding gases in the exhaust system. At a constant speed of operation, turbos increase thermal efficiency and fuel economy, especially in diesel engines.

[SaabJohan]

Ceramic coatings aren't used in the combustion chambers in a otto engine since they also reduce cooling and therefore increase the risk for knocking. They have limited use in diesels where the gains also are small. The coatings also need to be thick if they are going to make any difference which causes reliability problems as the coating can crack.

A modern otto engine have a thermal efficiency of some above 30% at peak (it can be calculated from the specific fuel consumption).
Diesels are more efficient, especially when they are turbocharged and intercooled. Turbocompound can also increase fuel efficiency, a diesel with all this can be 40-45% efficient or even better with large cylinders.

The problem with the otto engine is that the efficiency drops significantly on part throttle, it can drop to values as low as 10-15%.

For an otto engine the ideal cycle is a constant volume cycle while the diesel is a constant pressure or combined constant pressure/volume cycle, so they can not be directly compared.

A problem with the piston engines is that their expansion ratio is equal to their compression ratio, if the expansion ratio can be made greater the efficiency will increase.


a graph over brake specific fuel consumption for an Opel engine

Car manufacturers spend quite some work to reduce the friction of their engine, this can be done by using lighter valves (preferbly 4v/cyl) and softer valve springs. Friction is also being reduced by using smaller bearings, in racing often only one piston ring + oil control ring is used as it reduces friction, coatings like Grafal on the pistons and Nikasil on the liners and so on.

[TechX]

I was lucky enough to talk with an engine builder who worked with Nascar engine builders. It was like a 5 hour conversation and I wish I took his # or recorded what he said. I remember him saying that they used different coatings for different applications on the engine. Racers take any little edge they can get and he talked as though the biggest benefit was with reliability for the most part. It just made sense the way he explained coatings that the engine was more resistant to heat/friction in extreme conditions. I also found the fact that the cam lobes are all different interesting.

[Evil Result]

Well... any engine designed properly with Heat resistant, friction reducing coatings will have the added benefit of more HP because more of the thermal energy will transformed into mechanical energy then being absorbed by the cooling system, less fuel consumption, increased durbility, and reduced friction.

thermal coating are much more relible than in the past. since they seem to work well in Nascar