Future car engines

Posted on July 21st, 2008 in diesel,Driving Emotion,Economy,electric by Julian Edgar

Hybrid car drivelines can be characterised as being of series or parallel designs.

In a parallel hybrid, either of the two power sources can drive the wheels. In a petrol/electric parallel hybrid, that means either the petrol engine, or the electric motor, can propel the car.

In a series hybrid, one power source drives the other that in turn propels the car. For example, in a diesel electric series hybrid, the diesel motor might drive a generator that charges batteries. These batteries in turn power the electric motor that pushes the car along.

The best known of all hybrid cars, the Toyota Prius, uses a series/parallel design; most of the time the electric motor and the petrol engine drive the wheels directly, although the petrol engine can be used to drive a generator that in turn charges the battery pack.

Series hybrids have always had efficiency problems: the number of steps in the process – especially if that process involves charging and then discharging a battery – reduce overall efficiency.  However, the major benefit of a series hybrid is that that the engine can be run constantly at its peak efficiency, acting as a small power station.

New battery technology also means that more of the energy you put into the battery can now be extracted from it.

The Chevrolet Volt is one example of an upcoming series hybrid.

If series hybrids start to take off, the search will be underway for very efficient engines that can be run continuously at one speed, allowing the production of electrical power at the highest efficiencies. Because the engine can be optimised for a single speed, many of the traditional design constraints of car engines disappear. For example, the intake, exhaust and camshaft tuning can all be optimised for that one engine speed.

Engines other than internal combustion designs may also become attractive. The external combustion Stirling Cycle engine is one such design. One commercial application of this engine design can be found in the WhisperGen boat generator.

The WhisperGen power source is an electronically-controlled, four cylinder, diesel-fuelled Stirling engine. A wobble yoke is used to connect the pistons to a common shaft, an approach which reduces side loadings on the dry piston seals and guides. In addition to DC power, the Stirling engine design yields a plentiful supply of hot water at up to 75 degrees C; the company claims that over 90 per cent of the fuel energy is converted into heat and power.

The motor/generator system is extraordinarily quiet, with a claimed sound output of 55dB(A) at 1 metre. As the company points out, that’s quieter than normal conversation!

Maximum outputs of both the 12V and 24V versions are 800W (electrical) and 5.5kW (thermal). Fuel consumption is 0.75 litres/hour at maximum output. Mass of the unit is 90kg and its dimensions are 450 x 500 x 650mm.

Clearly this unit is optimised for its boating application; a very small and lightweight hybrid car would need less hot water and more electrical power! However, it is potentially a glimpse of the future engines that may be considered for series hybrid cars.

11 Responses to 'Future car engines'

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  1. Simon Bain said,

    on July 21st, 2008 at 7:18 pm


    Take a look at this web site http://www.octafuel.com. They have been advertising on TV in NZ. I think a retofitted hybrid fuel cell is right up your alley. I will be intrested on your comments.


  2. Gordon Drennan said,

    on July 22nd, 2008 at 12:05 am

    An engine that produces 5.5 Kw of heat to produce 800 watts of electricity is only efficient if you need the heat. If you don’t its godawful terrible. I would think they are selling it not on its “efficiency” but on the fact that its quiet.

  3. Henri said,

    on July 22nd, 2008 at 10:26 am

    Julian, Clessie Cummins was working on a 3 cyl version of this engine in his basement shop, the day that he died; about age 96 if I recall correctly.

  4. Mitchell said,

    on July 22nd, 2008 at 1:34 pm

    What about the ultimate evolution of the internal combustion engine- Electronic Camless Valve Control? When can we expect this to come to market?
    Other things such as HCCI, 6 stroke ( with steam) should be in every new car these days, engine technology seems to have stagnated for the past 5-10 years ( direct injection is becoming more prevalent but that was pioneered much earlier).

  5. jan said,

    on July 22nd, 2008 at 6:46 pm

    I believe there was an experimental stirling engined car in the 70’s or 80’s by a major manifacturer, can’t remember the details. The only downside was it required a (relative short) warm up period, but otherwise it worked. However a stirling-electric hybrid would be ready to drive on electric power right away.

    Also i wonder if there’s any merit in using a small stirling engine powered by cooling fluid to drive the alternator. This would make the petrol engine itself more efficient as energy “wasted” as heat suddenly becomes very useful, mechanical drag from the alternator can be removed from the engine, and perhaps (although the cooling requirements for the stirling engine may undo this) the radiator could be smaller = less aerodynamic drag
    I’m sure theres issues to overcome that might make the idea less suitable, but to me it seems using proven technology in a fresh way to make the sum of all things more efficient, is the way to go right now, while we look for even better more advanced solutions.

  6. Peter said,

    on July 24th, 2008 at 1:02 pm

    J, Can you give us a quick BSFC comparison between this Stirling engine and typical engines used in current hybrids?

  7. Julian Edgar said,

    on July 24th, 2008 at 5:04 pm

    Simon, it doesn’t look a very impressive website. perhaps they have better test informaton that they don’t put on their site…

    Gordon, I think I already wrote it needs to produce more power and less hot water.

    Peter, it’s a good question but I have no idea

  8. jan said,

    on July 24th, 2008 at 6:24 pm

    there’s an intersting entry on wikipedia on the stirling engine with a short bit on it’s automotive applications:


    the following quote is about a test where they made a comparison between a standard engine and a stirling engine in the same vehicle in the mid 80’s

    “the results show that the highway gas mileage was increased from 40 to 58 mpg and the urban mileage from 26 to 33 mpg with no change in gross weight of the vehicle. Start-up time in the NASA vehicle maxed out at 30 seconds, while Ford’s research vehicle used an internal electric heater to jump-start the vehicle started in only a few seconds.”

    perhaps the cost of a stirling engine are to great, but the figures make it seem a very valid power source for a car, especially since automotive technology should have progressed at least a bit since the 80’s.

  9. BG said,

    on July 25th, 2008 at 10:11 am

    Regarding the Octafuel concept – it would initially appear a reasonable concept that would work (pity there’s no more detail). However running a couple of (even very optimistic) numbers might show otherwise. If you used electricity generated using regen braking (and no friction braking), you recover ~30% of the vehicle energy. If the electrolysis to H2 is 50% efficient, add a loss for temporary storage etc say 70%.. results in maybe 10% fuel consumption improvement (at very best – 3% may be closer). So unless they show how it really works I wouldn’t be buying one- You’d save more fuel using water injection!

  10. BG said,

    on July 25th, 2008 at 10:20 am

    Oops I think I should have multiplied that 10% by the engine efficiency of say, 25% ie 2.5% fuel consumption improvement at best, so probably more like 0.6% fuel savings

  11. used cars uk said,

    on August 20th, 2010 at 1:14 am

    “An engine that produces 5.5 Kw of heat to produce 800 watts of electricity is only efficient if you need the heat.” You are spot on !