I can't be the only one who's ever thought of this, but obviously it hasn't caught on:

In terms of energy density, fossil fuels are the best thing around short of enriched uranium (and, flammability aside, a lot safer). The biggest two problems with using fossil fuel as an energy source are the low thermal efficiency of combustion, and the production of CO2 gases. The second problem is the more publicized, but it can be mitigated to a very large extent by solving the first.

One idea, which may catch on in industrial power generation, is the "carbon-conversion fuel cell", which at least in the lab gives a thermal efficiency of about 80% of the theoretical energy potential of the fuel. However, the energy density (1kW of generation capacity per square meter of fuel cell surface area) and the operating temperature (750*C) make it impractical at the small scale.

However, we currently have technologies that can improve efficiency of combustion engines, we just aren't using them. ICEs typically top out at about 30% thermal efficiency; the rest is lost, primarily as heat, some as kinetic energy of expanding gas. We even go so far as to build cars with water coolant loops and mufflers to more efficiently waste that heat and kinetic energy. Well, what if we instead harnessed that energy to do more work for us before it leaves the car? Imagine a parallel hybrid with two additional generators in addition to the one powered directly off the drivetrain of the engine. One would be powered by steam from the superheated coolant (anyone who's opened the radiator cap on a hot engine will tell you this system is perfectly capable of producing copious amounts of pressurized steam), and the other would work much like a turbocharger, using the pressurized exhaust to turn a turbine, but instead of an intake compressor the turbine would power another generator.

To my knowledge, no car manufacturer has ever fielded a production passenger vehicle that utilizes these additional energy sources to charge the battery system. But these technologies are decades old; one would think that we have the technology to pass the breakeven point on power to weight with these systems. Does anyone have documentation showing these additional avenues of energy capture have at least been considered, and if so, why they're not being pursued more aggressively?

  • $\begingroup$ This has been flagged as off topic, and I agree (note that my answer below claims that it is an engineering and economic problem). But we have entertained some vaguely similar questions before in realms like aerospace. Opinions? $\endgroup$ Commented Apr 2, 2013 at 19:15
  • $\begingroup$ @dmckee I flagged the question. I think it's ok with questions on automotive and aeospace topics, but I think it should be questions that can be answered, at least partially, with physics. This seemed to be more concerned with the motivations of the auto industry. But uou never know, it might be possible to explain the whole thing with a short thermodynamic calculation. I don't mind personally if the question is kept open. $\endgroup$
    – jkej
    Commented Apr 2, 2013 at 20:53
  • $\begingroup$ @dmckee it doesn't strike me as an obvious case for closure, but I do think this could at least stand to be rephrased a bit to ask about the underlying physics more directly. If this were to be closed in its current form, I wouldn't object. $\endgroup$
    – David Z
    Commented Apr 3, 2013 at 4:03
  • $\begingroup$ 1935 William Still developed a combination IC/Steam engine, tested by the USN, concluded with fuel at $.19/gal, didn't justify the additional mechanical complications. BMW Turbosteamer greencarcongress.com/2008/04/study-steam-hyb.html $\endgroup$ Commented Apr 5, 2013 at 22:10
  • $\begingroup$ Steam “Waste Heat Recovery” cyclonepower.com/works.html www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2009/session5/… $\endgroup$ Commented Apr 5, 2013 at 22:11

2 Answers 2


What you are talking about is called a combined cycle engine. They are commonplace in stationary power generation, i.e. utility-scale electricity generation. There has even been some talk of combined cycle engines in cars.

As pointed out in the answer by dmckee, the reason this hasn't been widely applied in cars is that no one has demonstrated an economically competitive combined-cycle car. I promise you, if such a thing can pay for itself in gas savings then it will eventually be built and sold, unless some better technology makes it irrelevant.

In general there are many reasonable ideas that are physically permissible but economically or technically difficult or nonviable. You are effectively suggesting to add a steam engine to a car, which is quite a difficult proposal. I'd suggest that a hybrid gas-electric car is more economical than what you suggest, and even they have had a hard time catching on. In electric power generation it matters much less that the combined cycle engine has a larger sunk cost than a normal engine, is heavier, etc., so the economic balance works out.

Bringing the question back to physics, no matter what you use for heat scavenging, your engine including all of its "subengines" cannot exceed the Carnot efficiency corresponding to the largest temperature difference in the engine. Adding additional heat engines will help to approach the Carnot limit. In order to beat Carnot, you can't use heat as an intermediate step between chemical energy (fuel) and mechanical work.

  • 4
    $\begingroup$ Also, I have one comment: I don't agree that the CO$_2$ issue "can be mitigated to a very large extent by solving the first." It certainly would help, however even if every engine were doubled in efficiency, who's to say that wouldn't drive down the demand and therefore cost of gas, resulting in more, larger cars on the road? This falls outside the realm of physics and the scope of this site. $\endgroup$ Commented Apr 3, 2013 at 3:22
  • $\begingroup$ Wile the last paragraph is true in principle, the motor exhaust is in practice often far hotter than ambient temperature. Using the waste heat means that the lower Temp is lower and the total temperature difference utilized, thus the theoretical max efficiency is higher. $\endgroup$
    – mart
    Commented Apr 4, 2013 at 11:03
  • $\begingroup$ @mart It depends on what you consider the theoretical max efficiency. Lower temperature exhaust is necessary to approach the Carnot efficiency for the temperature difference between the cylinders and environment, which does not depend on the temperature of the exhaust. Of course, the Carnot efficiency between the cylinders and the exhaust does increase as the exhaust temperature decreases. $\endgroup$ Commented Apr 4, 2013 at 11:44
  • $\begingroup$ My point is that the carnot limit for the whole engine incl sub-engines for efficiency is higher with exhaust heat utilization than without. $\endgroup$
    – mart
    Commented Apr 4, 2013 at 12:42
  • $\begingroup$ @mart That's not true though! The Carnot limit for the whole engine is $\eta = 1-T_c/T_h$ where $T_c$ and $T_h$ are the lowest and highest temperatures anywhere in the engine, respectively. Combined-cycle will help you approach, but not increase, this limit. $\endgroup$ Commented Apr 4, 2013 at 12:48

Recall that in a car you have not only the extra capital cost and extra maintenance (which all power system must be concerned about) but must also carry the extra weight of and find the extra volume for the additional system. And this being an engineering question, not a physics one you must then ask "Is this worth it?"

Recall also that the extra weight probably means beefing up the suspension (which adds more weight) or reducing the working load.

Recall that extra volume may mean making the car bigger (and therefore heavier) or less streamlined.

Recall that if you deploy novel systems that have an effect (or in this litigious times) even a perceived effect on safety there will be cost and maybe fewer sales.

Recall that each of those additional systems runs on a much smaller temperature difference than the primary IC engine and therefore has a lower theoretical maximum efficiency.

Our cars have traditionally run on IC because you get a lot of power per unit weight and volume, not because these are maximally efficient systems. In the early days of motoring steam and electric were both tried. They lost the economic battle.

  • $\begingroup$ Valid concerns, but note, however, that wood gas generating systems were added to vehicles before (especially during war time, when the fuel availability was scarce). While it did impact all aforementioned areas, it was considered viable enough to implement given trade-offs. Important difference considering maintenance, however — gas generators have essentially no moving parts, so that helps. $\endgroup$
    – theUg
    Commented Apr 15, 2013 at 15:53
  • $\begingroup$ @theUg I've heard that coconuts are popular in the same application in poor tropical areas, but these systems are typically removed once fuel availability because reliable. As always cost and availability of materials fold into engineering decisions. $\endgroup$ Commented Apr 15, 2013 at 15:59

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