You guys are way oversimplifying. There are far more variables than just the number of cylinders involved. There’s chamber head shape, that affects not only wave propogation but also how much valve area can be created (angles at the top allow larger valves), theres how many valves can be stuffed in (four smaller circles creat more valve area than three in a given cylinder size, there’s compression, stroke (a long stroke is pushing a longer lever at the crank, which has advantages and disadvantages), and a whole lot of other factors. There’s also the crank angle used…a flat crank has different characteristoocs than, for example, a 90% crank.
My reply was meant to be oversimplified and viewed as an “on paper” data point. When we did those classroom exercises, I remember holding the many variables constant across the engines we were comparing. In real life, I agree there are way too many variables used in practice to make any meaningful comparison.
If you enjoy British humor, here’s a video on the Triumph Rocket III. Warning!! You might spray coffee all over your computer’s keyboard through your nose while watching it so put the coffee down and make sure the coffee in your mouth has been safely swallowed.
“2: We spent a whole class comparing an 049 model airplane engine to an ocean liner engine (whose piston travel is 2 stories high and has 10 ft diameter pistons). The two engines had remarkably similar brake specific fuel consumption, efficiency, and horse power per unit of engine displacement. They were not identical, but close enough to realize both engines were dealing with the same constraints of physics.”
Those old Cox .049 engines were awfully thirsty for their size and power, even when you consider that their fuel was about 18% oil and methanol only has 9600 BTU/pound and nitromethane only yields 5000 BTU/pound.
In favor of small engines, the high rpm they are capable of means that the hot high pressure gas in the combustion chamber gets expanded before it has time to cool off through the engines high surface area to volume ratio.
A Harley Davidson V-twin has a low surface area for an engine that displaces 88 cubic inches but a 600cc four cylinder motorcycle engine has a low surface area for an engine that can crank out 100 horsepower.
The Harley engine will be efficient at low rpms but low rpms will kill the 600 four cylinder engine’s thermodynamic efficiency.
A throttled Cox .049 won’t even idle below 5000 rpm.
Thanks B.L.E…
If it hadn’t been such a piece of junk it actually would have been a great little car. The styling, handling, operation, and utility (hatchback) were great for 1972. It was fun to drive when it was new. It’s just that every single piiece was a cheap as could possibly be made, every design shortcut imaginable was incorporated into the car, and everything just kept breaking. I dumped it when the rear axle came apart. The whole axle with wheel slid right out of the housing.
My wife’s Honda Element has a 2.4 liter four, five fit in it and I would not describe it as “grossly underpowered”. The engine actually puts out more horsepower than a lot of the sixes and small V-8’s from the “good ol’ days” used to in the '60s.
Today’s 4’s are as powerful as 6’s that are 10 years old. Our Silhouette has 195 HP. Surely a 4 can do that now. Power is adequate in our van. Not spritely, but adequate.
I was eyeing BMW’s new 1600cc six cylinder motorcycle at the local dealer yesterday. It seems to come with everything but auto-pilot and for only $27,000 plus change, one of them can be yours.
I’m just afraid that if I owned one and I twisted the loud handle open, cops might mistake me for a jerk.
B.L.E., keep in mind that every new BMW motorcycle I have seen requires high octane fuel. I suppose if you can afford a BMW, you shouldn’t bat an eye at expensive fuel.
Personally, I love the roar of a v-twin, even with the stock exhaust. It doesn’t have to be loud to sound good.
Whitey, there is actually a very practical side to this new BMW, it has a single throttle body instead of a separate carburettor or throttle body for each cylinder. Wow! What took them so long to figure out that only one cylinder is using the carburettor or throttle body at a time?
I hope this catches on and is incorporated into some of the fours. Each cylinder needing its own carb has been one of motorcycling’s most enduring sacred cows and for no good reason in my opinion.
