3 vs 4 cylinder

[guitarterry]

a 48 cylinder motorcycle (just for laughs)
48 cylinder motorcycle

[guitarterry]

hey dcb, when are u swaping the 250 2 cylinder for a one cylinder bike

[dcb]

I've considered swapping engines for a slightly smaller one cylinder. Hadn't really thought about the reasons why it would be more efficient till you asked. I'd rather go electric though

FYI, more cylinders can make an engine LESS dependable/reliable because there are more moving parts.


And there are 3 cylinders that have had the same gearing fitted as the 4 cylinder metro, and thus they have the SAME cruise rpm. And I'm guessing they cruise more efficiently because they are smaller displacement and have less cylinders.

[basjoos]

Originally Posted by dcb

This is a little hard to explain, and I have to make some generalizations...

If you hold displacement and stroke equal, an engine with less cylinders will be more fuel efficient. The ideal (and the limit) being one cylinder, where the shape of the total piston area is a single circle.

Lets say that you have a 2 cylinder engine with pistons of radius 1, the total circumference around both pistons is 12.57.

Whereas if you have a single piston engine with the same stroke/displacement it would have a piston radius of 1.414 and a total circumference of 8.88.

This means that the single piston engine has about 1/3 less contact with the cylinder wall, and thus significantly less friction, even though the stroke and displacement are identical.


Also consider that less pistons means less connecting rods and bearings, and less valvetrain and camshaft lobes.


And if we are comparing a larger 4 cylinder to a smaller 3 cylinder, the larger 4 cylinder is going to have more pumping losses for a given power output

Also fewer cylinders gives a better ratio of cylinder volume to engine block volume and less cylinder surface area to heat up per cc of displacement, so a 1 cylinder motor is much quicker to warm up to operating temps after a cold start than a V-8 of equal displacement.

[metromizer]

If the 4 cylinder Susuki engine is truely superior, you could keep all 4 cylinders and destroke the crank to reduce displacement to l1. That might get you closer to the 3 cylinder's mpg numbers, but the cost would be huge downside.

At a minimum, offset grinding the crank and rebalance ~ $400, semi-custom pistons w/greater compression height <move the pin down the skirt> $500 or custom length rods for a Suzuki, ~ $200ea. You might get lucky if you search high and low, and find a slightly longer OEM rod from a similar engine that could be altered slightly to work, but my experience with mixing and matching rods and stroke regrinding crank journals has been 'it's real time consuming' and not real cost effective.

[guitarterry]

Originally Posted by dcb

.

Lets say that you have a 2 cylinder engine with pistons of radius 1, the total circumference around both pistons is 12.57.

Whereas if you have a single piston engine with the same stroke/displacement it would have a piston radius of 1.414 and a total circumference of 8.88.

This means that the single piston engine has about 1/3 less contact with the cylinder wall, and thus significantly less friction, even though the stroke and displacement are identical.

This is wrong, wrong, wrong. again apples to oranges. if u turn the one cylinder with a area twice in a minute. that would give u a total of 17.76. while the two cylinder turned once is still 12.57. here is what we are doing. a three banger turned 1.3 times in a minute would = the area, friction, and everything else, BUT the alt, water pump, etc would turn slower. plus frictional losses increase with the square of speed. Both would fire 4 times a minute. AGAIN WE ARE TURNING THE ENGINES AT A DIFFERENT SPEEDS TO MAKE FRICTION EQUAL.

[dcb]

Dude!! You need to try again

[guitarterry]

Originally Posted by metromizer

At a minimum, offset grinding the crank and rebalance ~ $400, semi-custom pistons w/greater compression height <move the pin down the skirt> $500 or custom length rods for a Suzuki, ~ $200ea. You might get lucky if you search high and low, and find a slightly longer OEM rod from a similar engine that could be altered slightly to work, but my experience with mixing and matching rods and stroke regrinding crank journals has been 'it's real time consuming' and not real cost effective.

If u actually did offset grind and destroke and use a xfi cam with less lift. couldnt u just mill the head to boost the compression back up. actually if u maxed the compression that would help out even more. even easier would be mill the block. of course u would need to use oversize bearings

[guitarterry]

Originally Posted by dcb

FYI, more cylinders can make an engine LESS dependable/reliable because there are more moving parts.


And there are 3 cylinders that have had the same gearing fitted as the 4 cylinder metro, and thus they have the SAME cruise rpm. And I'm guessing they cruise more efficiently because they are smaller displacement and have less cylinders.

Does anybody try to get the point. We are changing the gearing, or even the tires to turn the 4 banger motor slower. the same amount of cylinders is firing a minute. the reliability comes from a 4 turning at 2400 rpm and a 3 turning 3200 rpm. they are doing the same job but which would last longer. the effeciency comes from now turning the accesories slower like an underdrive pulley would. I do really like the destroke idea, along with the xfi cam (which would go in the engine evenwithout a destroke).

[guitarterry]

Originally Posted by dcb

Dude!! You need to try again

i cant tell if you are being sarcastic. i see the smile but just making sure.