Motorcycle vs car engine

[FXSTi]

Originally Posted by euromodder

All things aren't equal here

If you want to develop a high-MPG vehicle in reverse-trike fashion, I'd think it'd be a lot lighter than a car.

If you can make it light enough so that a 125 or 250cc FI engine becomes an option, it'd beat a small car engine on displacement alone .
(The smallest FI car engines you'll find are likely 1L - I doubt the 660cc Kei cars made it to the US ?)

There are a few of the 660cc Kei trucks running around. They import them and people use them like ATV's. $6,000 will get you one, but that seems a little pricy for an engine. I found an engine, trans, ecu and all for $1,200 on JDM Engine Depot

Kirk

[Ken Fry]

No over-analyzing: Car. For sure.
Car .4lb/hp-hr
Bike .5lb/hp-hr

[dcb]

are you including the transmission in the bike figure?

[dcb]

one of the fastest "production" cars: SSC_Ultimate_Aero_TT
9.9 second 1/4 mile
cost 1/4 million and up
availability, very hard to get your hands on, price goes up to 1/2 million $.

one of the fastest production motorcycles: Ninja ZX-14
9.04 second 1/4 mile with rider
cost 10k and up new.
available everywhere.

honorable mention: Hayabusa @ 9.23 seconds

given acceleration is largely a function of power to weight, you might want to do a *little* more analysis Ken

[Ken Fry]

Originally Posted by dcb

given acceleration is largely a function of power to weight, you might want to do a *little* more analysis Ken

I gather that you misread my post. The OP is not asking about power-to-weight ratios, but instead about engine fuel efficiency, which is expressed (in the US) in pounds of fuel consumed per hp-hour.

Motorcycle engines (excluding Harley engines, which are better suited to lawn mower use) have been developed mainly for high specific power output (high hp per cc). Car engines, however (partly because of much tighter emission regulations and some market demand for fuel efficiency) have been developed for high specific efficiency (i.e., low brake specific fuel consumption, BSFC). Even motorcycle engines, however, are better than they were when two strokes were on the road. Many two strokes consumed .6 lb/hp-hour. Now, motorcycles are around .5, although, as Oldmechanic pointed out, some are around .45.

Expressed as efficiency in percent .5lb/hp-hr is 25%. Motorcycle engines and general aviation engines fall into this area. .4lb is about 31% efficient. Most current car engines are pretty close to this. .35lb/hp-hr is about 36% efficient, just a little lower efficiency than the Prius engine. (The Prius engine sacrifices specific power output and low speed torque to gain this efficiency level.)

All these are peak efficiencies. Because variable valve timing is common in cars and rare in motorcycles, the off-peak efficiency differences are even greater. Unless you have an actual BSFC maps in front of you, a simplified method of getting a feel for consumption vs rpm is to take the dyno torque curve and turn it upside down. Motorcycle engines tend to be peakier, with narrower torque curves. Their efficiencies also tend to fall off more quickly when they are away from peak torque.

If the engine is installed in something other than a series hybrid, then the off-peak efficiencies become especially important, because in non-hybrids, engines spend most of their time far away from the peak efficiency areas. Thus the often quoted 15% (or so) actual operating efficiency of the typical car engine.

As an old motorcycle racer, I definitely agree that motorcycles offer far more fun per dollar than cars.

[Ken Fry]

Originally Posted by drmiller100

nope.

shafts are VERY efficient. Constant Velocity joints are VERY efficient. Non O-Ring chains are VERY efficient, as are O-Ring chains.

Apparently you did not read Franks comment, yet quoted it. Odd.

Frank is correct: he stated "Power transmission efficiency drops at least 1% with every chain, belt, or gear mesh between the crank and the wheel(s) right?"

That is precisely correct. If you don't believe that, then you'd have to provide some test results that show otherwise to gain any credibility. Chain manufacturers, who have an incentive to claim high efficiency, do not claim more than 99% efficiency for chains. It is a very rare straight cut gear mesh that is better than 98% efficient. Helical cut are worse, and hypoid gears are worse yet. Toothed belts are advertised as being as much as 98% efficient, but never over 99% efficient.

Math counts. Frank is right.

[redpoint5]

Originally Posted by Ken Fry

(...Harley engines, which are better suited to lawn mower use)

No no, people want reliable lawnmowers that don't trail oil everywhere they've been.

[jamesqf]

Originally Posted by redpoint5

No no, people want reliable lawnmowers that don't trail oil everywhere they've been.

Not to mention that it's likely to attract a mob of sledgehammer-wielding neighbors :-)

AFAIK, the only good use for a Harley engine is as scrap metal.

[Ken Fry]

Originally Posted by dcb

are you including the transmission in the bike figure?

I'm not sure who this was directed to, but if it was to me, then no, the transmission is included in neither of the figures I gave. These are figures from engine labs where there is nothing between the engine and the dyno: typically just a straight coupling connecting engine and a hydraulic pump or generator.

Transmission losses depend upon the exact organization of the transmission. In the old days of car transmissions, top gear (third or fourth) was "straight through," so there were no loaded gear meshes in the transmission. A little oil was flung around, and there was some bearing drag, but the transmission was very efficient in top gear. In the other gears, power flowed through at least two loaded meshes, (typically jumping from the input shaft to a layshaft and back to the output shaft, so efficiency was lower.)

Overdrive typically adds a gear mesh, and the consequent losses. However the increased loading and speed reduction on the engine (and lower BSFC) more than compensates for the gear losses.

Motorcycle transmissions can be in some ways efficient: some use straight cut gears which are noisy but efficient. But Franks point is correct, that many motorcycles use a lot of gear meshes that are absent in a car transmission. His Gold Wing is incredibly lossy in this respect. (Where this gets almost ridiculous is in bikes that have the crankshaft oriented transversly, but have shaft drive: this entails two lossy right-angle turns.) Most old style bikes mount the sprocket driving the chain to the rear wheel on a counter shaft, which means that there are always two loaded gear meshes in play.

There were a few bikes that in top gear had from the engine a oil bathed chain primary (about 98% efficient) a straight thru gearbox (99% eff, with losses mainly from bearings and oil flinging) and a chain final (98%, if perfectly clean and well lubed.) No conventional car beats this, because the losses in a hypoid final drive alone are the same (or slightly higher) than these.

But there are all sorts of exceptions. The transmission (and rear end if there is one) are separate issues about which there are few applicable generalities. Even in cars, there are chain final drives (highly efficient) helical gear final drives (quite efficient) and hypoid gears (lossy, in comparison). (But lossy means only 5% or 6% loss, often.)

Then there are weight issues... but that gets outside the simple answer the OP sought, I suppose.

[Ken Fry]

Originally Posted by euromodder

All things aren't equal here

If you can make it light enough so that a 125 or 250cc FI engine becomes an option, it'd beat a small car engine on displacement alone .
(The smallest FI car engines you'll find are likely 1L - I doubt the 660cc Kei cars made it to the US ?)

For quick-and-dirty, a 250 cc motorscooter is hard to beat: the engine, trans and rear wheel are all one module (and there are no shift and clutch controls to rig). The Zing! was first an enclosed two wheeler (which was lighter than the scooter from which I took the engine package). Then it was on three wheels, but still powered by the 50 cc scooter engine. It still had the scooter's top speed, but had gained enough weight with the additional wheels and car brakes, steering, etc. that acceleration was worse. But it pushed the thing around well enough for the intended purpose, which was to test the chassis. If I were not obsessive about fuel efficiency, and wanted to just drive around, a 250cc scooter engine would have made a nice little vehicle, at about 60-70 mpg.

Motorcycle and scooter tires are not designed to corner like a car tire, and swing arms are not designed for such loads either. Single-wheel-drive is not good in the north, where the drive wheel rides on the piles of snow and ice in the middle of the lane. Oversteer can be an issue, too, even on dry roads. But for simplicity, there is a lot to be said for just using the back half of a scooter or cycle.