The Persistence of the Piston Engine

Piston internal combustion engines are still the most effective at the time being despite advances in alternative propulsion methods. (Ducati/)

It’s clear from some of the comments appearing on this site that there are quite a few people who are impatient at how slowly the future of transportation is arriving. That’s only natural, as the hype celebrating battery power so easily outruns reality.

Therefore I ask myself, as I’m about to write something about motorbikes and their internal combustion (IC) engines, “Is this all irrelevant now, devoid of interest because it’s about to be swept away by the very next thing? Am I just a playback of tech writers of the 1930s, scribbling on about variable cutoff schemes for steam locomotives when diesel was the obvious future, boosting fuel efficiency by a factor of six?”

It’s not that simple. A while back I wrote a series of five articles for this site on electric vehicle propulsion: on electric motors, batteries, power supplies, charging systems, and component cooling. They provoked near-zero interest from our readers. No one at all wrote in saying “Gimme more! I thirst to hear about more efficient IGBT power transistors and I-squared R heating!”

Piston IC engines are still the norm and will be for quite some time.

Piston IC engines are still the norm and will be for quite some time. (Ducati/)

Therefore I think I should periodically review the several reasons why piston IC engines are still with us, still responding usefully to development, and likely to continue for quite a while.

  1. The world is tooled to produce, service, and supply consumables to the IC engine. This is a complex economic system; scrapping and replacing an existing economic system is a huge expense that responsible money managers seek to avoid. Scrap and replace 160,000 gas stations? Retrain 756,600 auto technicians (US Bureau of Labor Stats 2019 figure)? The replacement processes for steam-to-diesel and aircraft piston engine-to-turbine were driven, respectively, by strongly positive fuel and maintenance economics and by the Cold War. If anything, IC-to-electric suffers from negative economics for two major reasons: batteries remain expensive and electric-car makers understandably like their present upmarket customers. Tesla is the new Cadillac.<br/>
  2. For a time in the 1950s and ’60s the “mechanix magazines” (the print equivalents of today’s New Atlas and similar tech-enthusiast websites) told us to make ready for turbine cars, as Rover in England and Chrysler in the US showed prototypes. But no, the efficiency of gas turbines falls steeply as they are throttled to less than full power, and cars and bikes typically cruise on 15 to 25 percent power. Giving the turbine a regenerator, which recycles exhaust heat, reduced fuel consumption but made the power system quite large. The efficiency of the piston IC engine depends on its compression ratio, which doesn’t change with load. Cheap turbines were expected to appear as miracle ceramics replaced expensive jet engine superalloys, but it took decades longer than expected. As turbines are made smaller, loss from tip leakage around whirling turbine blades increases loss. Yes, the large fan engines on commercial aircraft have active tip clearance control, but those engines cost tens of millions apiece. And so turbine road vehicle engines were briefly hailed, then became a yawn.<br/>
  3. A favorite of tech writers is to compare the fuel efficiency of piston IC engines (between 25 and 42 percent, roughly) with the electrical efficiency of electric motors, which has been in the mid-to-high 90s for more than 100 years. Fuel efficiency and electrical efficiency are not the same, because that comparison leaves out the fuel efficiency of generating electricity. According to the US Energy Information Agency, 79 percent or more of US electricity in the first half of 2020 came from thermal plants, mostly burning natural gas, some coal, and the fissionable materials whose radioactive decay heats nuclear plants. Conventional thermal plants (coal, nuclear) offer roughly 35 percent efficiency, but the best of the new combined-cycle gas/steam turbine plants now coming online push 60 percent. The fast-starting simple-cycle gas turbine plants used to compensate for the on-again, off-again nature of wind and solar offer lower efficiencies in the range of 25–30 percent. Around 10 percent of generated electricity is consumed by transformer and line losses, and for electric vehicles there are the battery’s charge-discharge cycle and power supply efficiencies to consider, which are in the range of 75–85 percent and 90 percent respectively. Not surprisingly, when you work through all the energy transformations involved in the two systems—IC power and battery-electric—there is no clear advantage to either one. Yes, petroleum must be wrested from the earth, processed into fuels, and delivered to the user, but there are similar costs on the electrical side regarding the supply of natural gas, coal, and uranium.<br/>
  4. From time to time we are told promising power sources are “now being tested at Southwest Research Institute,” as if being tested were the same as being proven outstandingly efficient. Such power sources are also being shown to the usual possible investors. There have been new types of steam engines, superefficient Stirling-cycle engines, and marvelously compact barrel and rotary piston engines. All depend upon some combination of energy transformations. Chemical energy from fuels must be converted into heat and thence into pressure or velocity. Power from gas pressure or velocity must be converted into mechanical power such as a rotating shaft. Electrical power must be transmitted through resistances and processed into waveforms that can drive efficient electric motors. Each transformation involves some loss. Then the question is, can a hoped-for gain in efficiency pay the costs of converting the world to the new system?<br/>

One thing we know: There will be surprises. At present, the possibility of commercial fusion power is attracting fresh attention, just as is commercial space flight. Will private capital and the profit motive do the trick of unleashing unlimited electric power? Will inexpensive and safe battery systems enable wind and solar to power the world day and night? Let’s live long and see how it all plays out.


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