In another thread there was a discussion of geo metros, I mentioned how my brother used to own one and one of the 3 cylinders failed and stopped working.
The car originally had a 1.0L engine, 55hp, 3 cylinders.
So if one of those cylinders fails, does that mean the power is cut by 1/3 or is it more complex than that? Would the car suddenly become 37hp, or is the power output calculated differently?
The metro is supposed to go 0-60 in about 15 seconds with a top speed of 99mph. After the cylinder failed though the metro did 0-60 in about 1 minute and its top speed was around ~60mph.
So, in theory having one cylinder “fail” (I assume by that meaning that it does not get good compression or effective combustion) you theoretically lose that fraction of total power. In reality, the power throughput is an average over an entire cycle at some peak output, and having one more more cylinders fail means that it probably acts to further sap away power during the portion of the cycle it would normally be firing, so if you actually measured brake horsepower on a dyanometer it would probably be even less than 66% at theoretical peak output.
For what it is worth, I doubt a Geo Metro has ever gone 99 mph unless it drove off a cliff, and could get a 0-60 in 15 seconds only if it was being pushed by a semi.
It’s a lot more than 1/3 loss of power. You can measure the output power on a dynamometer to figure out the actual loss of power but I’m guessing more than 50%. You’ve got a bunch of things that decrease the power of the motor. Compression of the gas mixture in the cylinder, friction of the piston in the cylinder, mass of the piston, connecting rod, and crankshaft. You might be able to limp around but you wouldn’t be able to travel at anywhere close to normal high speeds.
Back in the day, I think it was GM who built a V8 engine that shut down half the bank of cylinders to make the engine more efficient during low speeds and idle, but they stopped the fuel injection and I think they stopped the valves from closing so that compression was less of an issue. I’m pretty sure that that concept never caught on though.
Measuring compression in an engine is a standard diagnostic but normally you just have shitty compression in a cylinder or two and the engine still runs but not nearly as well.
You’re probably pretty close as a first order guess. There have been engines designed to operate just this way starting with the Cadillac V8-6-4 in the 1980s. There are several on the market right now. Exactly how much power is lost depends on how the shutdown is handled in respect to pumping losses. The V8-6-4 just disengaged the rocker arms and let the piston pump in a closed cylinder. There might be better efficiency if the exhaust valve was open to prevent pressurizing the cylinder each time. It would probably make unacceptable noise, though.
And finally, back in the day it was a thing to convert every other cylinder in an engine into a compressor by removing the rockers and installing check valves.
True, but that means the flywheel is losing momentum during that lost stroke, so when the power stroke from the next cylinder comes, it is moving slower than it was designed to, and of course it only provides power on every second stroke. Frankly, with a three cylinder, four stroke engine, I’m surprised it could completely lose a cylinder and not shake itself to death.
Well, to make a compressor. A rather powerful one. One I saw was just an engine on two wheels and a tongue to tow it around. Ingersoll Rand made a really neat 6 cylinder radial engine/compressor like an aircraft engine. Really cool looking.
In my experience it depends how the cylinder fails. I once had a spark plug literally disintegrate on a V-6; the socket remained intact and the cylinder maintained its compression cycle but never fired. In that situation the power loss was very significant, certainly far more than the 1/6 you would expect. It was down to about half of normal power or less, sounded like a particularly load chugging steam engine, and barely made it to the repair shop! It’s like the absence of a power stroke when one was expected really upset the engine’s balance in a big way, even though it was just one out of six cylinders.
I’d think that losing a cylinder would confuse the hell out of the engine control computer, possibly causing it to make adjustments that kills performance more than the expected 33%. Depends on exactly how the cylinder is “lost”, what sensors the computer uses, and the specific control algorithms used.
Correct, and if you lost all of those drag items along with the indicated power output of the cylinder, the net loss to your 3-cylinder engine’s net shaft power output would be pretty close to 1/3. But the reality is that those loss items are still around.
Imagine the engine makes 70 indicated hp (“indicated” refers to the mechanical work delivered from the piston to the crankshaft, without regard to whatever losses might be siphoning off some of that power). If it makes 55 brake hp (mechanical power available at the flywheel), then the friction/pumping power is 15 hp (75-55). Now cut fuel to one of the cylinders. Indicated power becomes 2/3 of what it was, or 46.7 hp, but friction/pumping power is still 15 hp. So brake power is 31.7 hp, which is 57.5% of full rated power.