The Evolution Of Internal Combustion: Thermal Energy Management

Edward Niedermeyer
by Edward Niedermeyer

With so much attention focused on next-next-gen, alt-energy auto technology, we enjoy highlighting the incremental changes that are making good old internal combustion engines more efficient. The latest evolution to show up on our radar screen is BMW’s development of a host of measures [via Green Car Congress] which it hopes will someday reduce the inefficiencies of cold starts. Perhaps the easiest way of reducing low-temperature, high-friction starts is to encase the engine to slow down the engine cooling process (as well as insulating components that might otherwise need to be cooled). In fact, BMW has shown that with encapsulation, a 176 degree operating-temperature engine can keep its temperature as high as 104 degrees after 12 hours. But good luck trying to change your oil when your engine is surrounded by thermal materials.

BMW is also looking at exhaust gas heat exhangers to provide quicker warm-ups and even interior heating. But heat is free in an ICE, you say? High-efficiency diesels are actually so efficient, that many manufacturers have had to include electrical heating elements to provide sufficient interior heat. As gas engines become more diesel-like and thermally efficient (possibly with the advent of HCCI), they too might need to look at using exhaust heat for cabin heating. In any case, this technology won’t improve test-cycle efficiency because the engine would already be warmed up, but BMW says modern diesels should see noticeable improvements in everyday driving.

Perhaps the most radical gizmo being developed by BMW is the thermoelectric generator, which the Bavarians are showcasing as a component in exhaust gas recirculation systems. By integrating a thermoelectric generator just downstream from the exhaust manifold, a thermoelectric generator can develop as much as 250 watts of energy (equal to about a 2 percent efficiency improvement) without interfering with exhaust gas recirculation cooling or pressure. The only shortcoming at this point is size, which (as you can see from the picture above) makes integration a bit awkward. BMW hopes to eventually use its thermoelectric generator on exhaust systems, where higher flow holds the potential for greater energy generation. In that future application, a thermoelectric generator could also be used to more rapidly heat up a catalytic converter, reducing cold-start emissions.

BMW’s recent technology may not be earth-shattering, nor immediately production-ready, but it shows that there’s more to the future of the automobile than battery chemistry or fuel cells.

Edward Niedermeyer
Edward Niedermeyer

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  • Don1967 Don1967 on Oct 15, 2009

    Greg & carve, Thank you for clarifying what was a "common misconception" on my part... what clued me in was the comment about two-stroke expansion chambers. I found this longer explanation, which pulls both of your points together: That said, the total percentage of heat energy recaptured by a turbocharger must be relatively small, and partially offset by the back pressure and frictional heat it generates. Is anyone prepared to argue that a 200-horsepower turbo-four puts out significantly less heat than a 200-horsepower normally aspirated V6?

  • Carve Carve on Oct 15, 2009

    Thanks for the reply, Don. If the turbo extracted power from the exhaust and added it to the crankshaft, then yes: they'd put less heat out the exhaust for a given power output. However, turbochargers are almost exclusivly used to compress intake air, which increases power instead of efficiency. The same heat in a smaller engine means higher temperature, which is then cooled by expanding through the turbo. Heat & even temperature output at the tailpipe of a 200 hp turbo 4 is therefore likely similar to a 200 hp V6 when both engines are making their rated 200 hp. The advantage of the turbo engine is that it's smaller & lighter, and it has reduced pumping losses while cruising. The advantage of the normally aspirated engine is that it has quicker, more predictible throttle response and is simpler. If the turbo was geared to provide power to the crankshaft directly rather than compressing intake air, exhaust temp would indeed be cooler. The effect of this added expansion would be similar in principal to an atkinson cycle engine, which expands the charge to a greater volume than the amount drawn in and compressed. The efficiency of heat engines is largely proportional to the temperature difference between the hot side (combustion fire) and cold side (ambient air). As the air in the engine is expanded, it cools off. Power output falls as the hot air continues to expand, and you need a much larger volume (bigger engine) to extract the leftover marginal heat. You'll often refer to this as "quality" of thermal energy. A small volume of high temperature air has a higher "quality" than a big volume of lower temperature air, even if the total thermal energy is the same. This is why automotive engines tend to be less thermally efficient than stationary engines and powerplants- they'd have to be enormous to keep extracting heat from the cooling combustion products, which requires a bigger, heavier car, which requires more power, and so on. Since you have to use more energy to move that enormous mass it really doesn't save you much fuel. This is also why you'll often see race cars belching fire out the exhaust- they're just using that first little bit of high-quality thermal energy to maxamize power output at the expense of fuel efficiency. Turbines are a relatively light and simple way to extract some of that marginal heat. Using them to compress intake air is a way to actually IMPROVE your power to weight ratio, so that's why they're used almost exclusively for that purpose. Still, they don't actually generate boost that often and it'd be nice to extract power from that marginal heat all the time. A turbo alternator would probably be the way to go (especially in a hybrid) since it wouldn't add any additional weight or complexity & would remove parasitic drag from the engine. The hard part though would be de-coupling the compressor from the turbine when you didn't want to make boost. Perhaps the best way is to have the turbo ONLY power the alternatore, and then have an electric supercharger.

  • 2manyvettes Tadge was at the Corvette Corral at the Rolex 24 hour sports car race at the end of January 2023. During the Q&A after his remarks someone stood up and told him "I will never buy an electric Corvette." His response? "I will never sell you an electric Corvette." Take that Fwiw.
  • Socrates77 They're pinching pennies for the investors like always, greed has turned GM into a joke of an old corporate American greed.
  • Analoggrotto looking at this takes me right back to the year when “CD-ROM” first entered public lexicon
  • Alan My comment just went into the cloud.I do believe its up to the workers and I also see some simplistic comments against unionisation. Most of these are driven by fear and insecurity, an atypical conservative trait.The US for a so called modern and wealthy country has poor industrial relation practices with little protection for the worker, so maybe unionisation will advance the US to a genuine modern nation that looks after its workers well being, standard of living, health and education.Determining pay is measured using skill level, training level and risk associated with the job. So, you can have a low skilled job with high risk and receive a good pay, or have a job with lots of training and the pay is so-so.Another issue is viability of a business. If you have a hot dog stall and want $5 a dog and people only want to pay $4 you will go broke. This is why imported vehicles are important so people can buy more affordable appliances to drive to and from work.Setting up a union is easier than setting up work conditions and pay.
  • El scotto I can get the speedometer from dad's 72 Ford truck back. I can't get dad back.