What is the average energy efficiency of automobiles by type?

Question

One gallon of gasoline contains roughly 130MJ/gallon. Based on this, what would be the industry average estimates for percent of energy efficiency of automobiles by type (sedan, truck, SUV, compact, etc.)? I'm sure these values vary greatly by vehicle make as well.

I would believe that there is potential for huge improvements (orders of magnitude) in efficiency based on how so much energy is lost in the forms of heat, noise, vibrations, etc. Hybrid synergy drives already help some by converting kinetic energy into electrical potential energy, doubling fuel efficiency in some cases. Perhaps some of the heat can boil water to work like a locomotive engine, adding more power. The expansion of water could also assist in air conditioning with virtually no excess energy use. There are so many possible ways for improvements, which brings me back to the question of just how efficient the internal combustion drive is by itself.

Please, only answer if you have knowledge of this field.

Answer 1

Vehicles have to trade efficiency for other considerations.  Putting a boiler on an IC engine as an example, might capture a lot of waste heat, but make the car much heavier, requiring more power.  Cars which are smaller (lighter) need less gas to move about.  Drag coefficients also have a big effect - if you note most really efficient cars avoid a flat rear end - that is a big drag waster - large trucks would be much more efficient if they could 'boat tail' the flat rear, but that makes for a much longer vehicle, and would cause other problems - backing, unloading, etc.  

The current internal combustion engine process is pretty inefficient, and because we add a bunch of environmental controls, that adds to the inefficiency.  I am not ready to trade smog controls for better gas mileage just yet.  Fuel cells and other energy conversion processes might be the ultimate answer.  (Hydrogen powered vehicles could use fuel cells which are quite efficient along with the synergy drive technology to get the performance up to par...)

Answer 2

The question appears to be how efficient the internal combustion engine is. The answer is approximately 32%. The most comprehensive measurements were actually performed back in the 50s and 60s by Caris or Kerley.

The efficiencies so measured are those of the pressure/volume work done on the piston. The actual efficiency delivered to the crank shaft is somewhat less than that.

An interesting observation from this work is that the most widely accepted theoretical model of internal combustion engines, i.e., the fuel-air model, indicates that the practical achieved efficiency is substantially less than that predicted by the theory. That is, the fuel-air model predicts the efficiency of an 8:1 compression ratio engine to be about 55% where the experimental value is around 32%.

Caris noted in his paper "A new look at high compression engines" that the heat balance in the measurements indicate that there is no "unintended heat loss" which has long been used to explain the variance of theory and experiment. In fact, enormous efforts have been made to improve ICE efficiency by insulating cylinder walls or using ceramic liners, etc. but with little effect.

A new theory has recently been proposed by Frank A. Tinker in a paper published in the International Journal of Energy Research that seems to explain the overestimation of ICE efficiency by accepted theory. This paper goes on to explain that the accepted theory is not wrong, it simply makes an assumption about the operating state of the engine. The paper identifies this (unstated) assumption and projects that engines may be designed such that the assumption will be realized thereby making internal combustion engines far more efficient.

If Tinker's theory is accurate, which is likely considering that its accuracy is unprecedented in predicting engine efficiencies for Otto, Brayton, and Stirling cycle engines, then it may be possible to exploit the discovery and increase ICE fuel efficiency by about 70%. This would increase fuel economy from about 25 MPG to 43 MPG without loss of performance.

Tinker claims in the referenced paper to be attempting to exploit the discovery in an internal combustion engine but he's published no further papers on the subject since November of 2007.