Well, to be honest, I am asking an impossible question, but I would be interested in hearing others' viewpoints and any references would be appreciated.
What we find is that we can make engines more efficient, but it always brings an attendant cost. Some have suggested that the limit of practical efficiency is about 60%. (Modern researchers, and Rudolph Diesel, have said it.) Today, people talk about going to even higher thermal efficiency than 60% (after removing all losses). Just getting to 60% can be a great challenge. In our work (which is theoretical), we think we see a sizeable increase in cost when the expected efficiency of a heat engine (including friction and heat loss) is greater than 60%.
What do you think? What is the "best" practical efficiency that a heat engine (internal combustion engine) can be expected to achieve after making our best efforts?
Hey, if we assume classical IC piston engines (excluding kinetic motion change and friction loses), the limiting factor is the second law of thermodynamic.
In the fundamental spectrum, we are dealing with "wasting" energy produced by combustion in form of heat. The movement of atoms is what generates "power" but also what limits it. Improvement can be made in the conversion phase, to utilize more pressure and more heat from combustion on the power stroke while eliminating any heat that is left (which is a lot) before the compression stroke.
There are few ways how to go around this, for example material of combustion chamber (and cylinder walls) from non-heat conductive material, strategies like water injection (maximizing pressure while eliminating heat) or of course more precise control of events such as injection/ignition timing etc., which can also be achieved thanks to reduced/controlled temperatures. That is why I would recommend stacking these technologies, which is how we can creep closer to the maximal limit.
That being said, I would be optimistic and even go as high as >78%. Future technology can take us to that point, but i doubt it will ever be higher. Shame as it is, conventional IC engines are inherently inefficient (motion change, high pressure = high temperatures, etc.).
This question is too broad. But to answer briefly, most IC piston engines used for road vehicles are at 20 to 30 %. Most IC engines used for electricity generation have efficiencies of 30 to 40 % for those operating at 1500 rpm and between 50 to 55 % for very low speed engines operating for long hours (350 to 1000 rpm). The very high efficiency large engines are very small in number and do not factor much in the overall fuel consumption. I have not known any with values higher than these.
Hi Ernest Rogers. To answer that, you'll need to start by reading about "Exergy." It is an old classical topic by now, but it gives the answer on the best attainable efficiency if the best of the best efforts have been made.
It is an interesting topic and the experts above have provided very insightful answers. To add to it, I feel that altering the engine design parameters to improve efficiency has its limitations and the law of diminishing returns will always play spoilsport at some point. An example is the compression ratio - the improvement we get in efficiency is marginal beyond a point and it will incur additional weight and cost. That is the reason why even in diesel engines, the CR is limited to around 20. Similarly, most of the other design factors will also become less attractive beyond a limit. One approach which has been shown to be very promising (although there will be limitations on this front as well) is to employ cogeneration / waste-heat recovery / bottoming cycles. This can harness some of the energy which will otherwise go waste (i.e. exhaust or coolant energy), thus improving the overall efficiency. One good example is turbo-compounding.
Please have a look on my New Formula of the Mechanical engines brake work:
https://www.researchgate.net/publication/352221791_The_Modeling_of_the_Alternative_Mechanical_Engines_the_mathematical_brake_work_and_the_mechanical_definition_of_the_availability
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