engine efficiency, mpg and such

[Kmehr]

Ok all you engineers out there, I wanted to start a discussion on engine thermal efficiency and mpg. Probably going to get into stuff way over my head, but this came to mind while looking at a Calico Coatings brochure the other day.

It is my understanding that piston driven engines are very thermally INefficient. Correct me if I am wrong here but most practical diesel engines are only about 50% thermally efficient.

That being said, and all other factors such as vehicle weight, cost, and aerodynamics aside, what sort of improvement can we make to our engines to improve thermal efficiency, and how effective would they be? In other words, how effective in improving mpg would things like ported heads, worked valves, completely ceramic coating the intake system, exhaust system, turbo, and all bearings, optimizing oil weight, optimizing stroke length be to be volumetrically efficient to the turbo size ect?

Theoretically speaking, if you had two engines which were both 100% thermally efficient, a 2L VW diesel, and our own 6.5L, wouldn't the 2L use about 1/3 of the fuel as the 6.5L?

Does and increase in thermal efficiency of say 5% translate into a fuel efficiency increase of 5%?

[DmaxMaverick]

This has been discussed here before, and at great length. It's been a while.

Efficiency, not only thermal, will never be beyond much what we have now, in good working powerplants. Any gains will be very minimal. Increasing thermal efficiency will never return the investment (assuming it is currently optimal). Increasing the efficiency will only return the increase, multiplied by it's resultant efficiency. For example, if you are currently experiencing an efficiency of 50%, and increase it by 5%, you will not result 55%. It will be more like 52%, because you only have a 50% efficiency rate to begin with (you can only multiply by your initial factor), and you will always lose some in translation. If your resultant efficiency is 55%, then your gain was about 10%. It's fuzzy math, but that's the way it works.

Insulating the gas path (in/out) serves little purpose, with the exception of the exhaust system between the cylinder and turbo. All else only relocates the excess heat (the thermal loss) to another place. Insulating the entire exhaust would only cause more heat to be expelled from the tail pipe.

Couple any of this with the monetary cost of any improvement, and it isn't likely you'll own the vehicle long enough, or live long enough (you or the vehicle), to see a return. The only profitable improvement is less pedals (accelerator and brakes), and smart driving habits.

[NH2112]

Basically it's not a matter of insulating the exhaust to prevent heat loss (except between cylinders and turbo as DmaxMaverick said), but a matter of reducing the temperature of the exhaust. Exhaust heat is nothing but wasted energy - energy from burned fuel that wasn't used to drive the piston downward - which is why a turbocharger driven by something that's being thrown out anyway is such a good idea in the first place. The lower the exhaust temp for a given amount of fuel being burned, the more energy (heat) from that fuel is being used to turn the crank.

Maybe the key lies not so much in trying to improve thermal efficiency, but in making use of waste heat to do things that either aren't done now, or are done by the engine and therefore are parasitic drag on the main purpose of the engine (moving the car, not charging a battery or pumping coolant.) I'm no engineer but it seems to me a Stirling engine could make use of the waste heat in the exhaust to do SOMETHING.

[rustyk]

Reciprocating internal combustion engines are in the 15%-18% thermal efficiency range. Diesels are slightly more efficient than gaso due to the greater compression ratio (which extracts more heat from the combustion).

The basic calculation for heat efficiency starts with measuring intake, combustion, and exhaust temperatures in degrees Kelvin. Displacement provides info on the mass (volume corrected for temp) of the gas from which the heat is extracted for each engine.

The Diesel and Otto (gaso) cycles are different. Gaso Miller and Sterling cycles are slightly more efficient.

[Kmehr]

SO our engines are pretty much as efficient as they are going to get?

[rustyk]

Yep - short of a materials breakthrough that would permit higher operating temps. Note that thermal efficiency is only one element of an engine's overall efficiency. Mechanical efficiency and the engine's effectiveness in breathing and combining the fuel-air mixture also play an important part.

Thermal Efficiency is just the major element - kind of the "sound barrier", that, barring a technological "miracle", will limit how much overall efficiency can be improved.

[DickWells]

Doesn't water injection constitute an increase in thermal efficiency? If so, do's the cost of an installation offset any gains?; refering to what DMAX said about any improvements.

Some time ago, I started a thread, here, on these same lines, and one of the things that got it going, was a thing I read in Road & Track about some of the things that manufacturers were doing in that regard. One thing that BMW was doing, was regaining some of the heat loss through some sort of capacitor set-up in the electrical system, (I I R C):o They (BMW) were regaining a fair amount of otherwise lost energy through that proccess, and using it to drive some of the electrical auxiliaries. Again, cost of an installation, vs, gains?

[DmaxMaverick]

It depends on what "gain" we are talking about. "Gain" could be anything from monetary to mere personal satisfaction.

Water injection will offer gain(s), under specific conditions, but a monetary gain will likely never be seen. The gain will come in the form of performance and/or personal satisfaction. If you are trying to shave a few degrees from your EGT, and you get it, then that's a gain. A water injection system isn't free, and won't install/maintain itself. Time spent on maintenance is a loss. The value placed on that, or any loss, is for you to determine. On the same note, you could get the same "gain" (lower EGT) at the expense of time. If your time is less valuable than the money, then your greatest return will come from just driving slower.

The same formula applies to any enhancement, beyond what may be considered "functional".