The graph shows the efficiency of typical automotive gas and diesel engines as they vary over their power range, and at 50% max rpm which is typical of actual driving conditions. Note that the "Bourque engine" is a sophisticated modern piston steam engine proposed by Dr. Robert Bourque (
Bourque Steam Engine). The purpose of presenting this is not to introduce his engine, but to illustrate how the efficiency of typical automotive engines vary significantly over their power range (especially gas engines). You will need to know some conversion factors to convert to efficiency figures (note that gallons are U.S.):
1 KWh = 3412 btu , 2.205 lbs = 1000 grams, 1 gallon gasoline weighs 6.22 lbs, 1 gallon diesel fuel weighs 6.94 lbs. Also, one gallon of gasoline has a lower heating value of about 115,400 btu, and one gallon of automotive diesel fuel has a lower heating value of about 128,700 btu.
The efficiency of the 1.6 liter gas engine over the Normal Operating Range shown varies from 15.4% to 26.4% with a mean value in the low 20's%. The peak efficiency of this engine corresponds to 29.5%. Of course, the speed is limited to 50% of max value in this graph, so peak efficiency might be higher with higher engine speeds. However, these lower engine speeds are representative of real world driving conditions. Note that the efficiency drops dramatically at outputs below the Normal Operating Range.
The efficiency of historical steam cars were as follows: Stanley Steamer (6-8%), White (10-12%), Doble (10-12%), Pritchard (12-15%). The Stanley saw about 8-10 mpg on kerosene. The Doble saw about 10-12 mpg on kerosene and was a very heavy automobile at 6000 pounds. The peak steam temperature in all cases here were under 800F. Furthermore, these engine systems saw extreme thermal losses due to poor insulation, poor heat exchange, and distributing steam a distance from a steam generator to the expander (except the Pritchard unit that put the expander right next to the steam generator and did a good job of insulation). Kerosene shows about 124,000 btu per U.S. gallon (lower heating value). My research shows that the Pritchard steam car (converted '63 Ford Falcon) achieved 22.5 mpg on kerosene at 50 mph. According to the graph, this corresponds to roughly 20.8% efficiency for the gas engine. This is about 40% higher than the efficiency of the Pritchard engine. Therefore, if the '63 Falcon were powered by a typical gas engine of today, then it would consume about 71% of the fuel energy of the Pritchard car at 50 mph. This corresponds to about 29.5 mpg on gasoline. Note that the Ford Falcon was known to achieve fuel economy in the low 30's (mpg) during actual road tests designed to demonstrate optimal fuel economy (it was a competition with other car models during the 1960's, and the Ford Falcon won with its stock engine). In other words, my analysis here took figures from disparate sources and shows that they agree fairly well.
Now, consider an engine (any engine) that can power an automobile and show optimal thermal efficiency in the Normal Operating Range. If a modernized steam engine can be had to show similar dynamics of peak efficiency in the Normal Operating Range and with little variation in efficiency over a very wide output, yet achieve efficiency in the mid-20's or higher , then it would show superior fuel economy to existing gas cars. An even match requires the efficiency to increase over the Pritchard engine by about 40%. That is, the efficiency has to get the low 20's%. Now, the Doble steam car is known to achieve fuel economy during city driving roughly the same as modern gas SUV's of similar weight (6000 pounds). This was due to the fact that the Doble does not use steam when stopped or while coasting. This same dynamic would also be favorable for a modernized steam car. That is, city driving would be much higher in a highly efficient modern steam car as compared to conventional automobiles, and this would take the combined cycle fuel economy higher.
In summary, the efficiency of a modern steam car has to be 21-23% to match the fuel economy of conventional modern gas cars. Furthermore, if one deducts the roughly 5% loss from the torque converter, then there is additional favor for a steam engine system with direct drive. There is also the matter of superior fuel economy in city driving that was mentioned before, and this should take the combined cycle MPG higher all else equal. I emphasize that when considering the merits of a modern steam car with respect to fuel economy, then emphasizing the peak efficiency of the engine is a red herring. Now, throw in the other advantages including inherently quiet operation, multifuel capacity with less fuel refining, no transmission, and inherently clean emissions, then I believe the rational position is to consider the prospect of a modern steam automobile as having some merit.
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