BBC presenter Jon Stewart had an article published online today that claims we need a new kind of internal combustion engine, because the Otto cycle engines in common use still have “similar” fuel economy numbers to the Model T.
here’s a link to it. go read it and come back. i’ll wait.
now, let’s examine his argument. first, i’ll compare the Model T (the most popular car of its time) to the Toyota Corolla (most popular car now), instead of his entirely hypothetical “modern car.” this will allow me to use actual figures, rather than generalizations of questionable utility.
Stewart states that the Model T had “a claimed fuel economy of 13-21 miles per gallon” and the typical modern car has “a slightly improved fuel economy of somewhere in the range of 20-30mpg.” the base or standard model US-market Corolla gets 27-35 mpg; using his numbers, modern vehicles get around 50% better fuel economy than the Model T. the Corolla gets slightly better than that. either way is hardly “slightly improved.”
but, according to Stewart, this is not enough. a fair enough assertion, actually, given the environmental state of the planet and the sheer number of vehicles on the road. but is it really the ‘fault’ of the Otto-cycle engine? is it really lacking in ‘efficiency’?
the Model T, according to Stewart, got 20 hp out of a 2.9 liter engine. that’s about 6.9 hp/liter. his hypothetical “modern” car lacks a defined displacement, but produces 200 hp. our base US Corolla gets 132 hp out of a 1.8, for 73 (and a bit) hp/liter. ten times the specific output would seem quite a bit more efficient, depending on how one measures efficiency.
which is where the author throws in a slightly silly curve ball. he points out (via a quote from an engineer shilling the revolutionary new engine we’re supposed to be switching to) that the “efficiency” of the internal combustion engine has been stuck at 33% for “100 years.” well, yes. and at the same time, no.
what this engineer appears to be referring to is thermal efficiency, an entirely different thing from (although not unrelated subject to) fuel efficiency. the Otto cycle (which is really what we’re talking about here) is, in practice, often a bit less than 33% thermally efficient. numbers in the mid 20-percent range are much more rational. these numbers are entirely normal in the real world, particularly given the compression ratios required for spark-ignition petroleum engines. an ‘ideal’ Otto-cycle engine, working in a (nonexistent) perfect environment would have to near 16:1 compression before it approached 70% efficiency. the Model T had a paltry 4.5:1 compression ratio, and our Corolla example has a 9.8:1 ratio. in other words, thermal efficiency is a less-than-entirely useful measuring stick.
where modern engines excel is in the maximum extraction of heat from fuel; that’s why they have a greater specific output and (only) 50% or so better fuel efficiency than the Model T. if your heat efficiency is stuck by physics at 30% or so (as it is), then you would be best served by increasing the amount of heat you get from a specific volume or mass of fuel. modern engines burn leaner and hotter than their predecessors; they use less fuel to produce more heat. they may function at the same level of heat efficiency, but the actual fuel efficiency is much improved.
and, perhaps worst of all, he ignores the elephant in the room of the fuel economy argument: weight. the Model T weighed a paltry 1200 pounds; the modern Toyota Corolla is over 2800. this directly impacts fuel economy (and should impact any discussion of real-world efficiency); the Toyota’s engine must perform 2.5 times the work of the Model T’s. even if we assume the best case for published fuel economy (in miles per gallon) for the Ford and worst case for the Toyota, the modern car is still getting over twice the amount of work done per unit of fuel than the older.
are there still places to make improvements? of course. but that takes baby steps based in real-world observation. unfortunately, the incremental improvement towards a mature technology is ignored or denigrated by Stewart. he seems to seek a eureka solution, some great leap forward. that’s not really how technology works, dude.
quote of the day:
“Statistics are no substitute for judgment.” – Henry Clay