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800cc Bike vs 800cc Car - fuel economy
Hi,
I just wrote an answer to An 800cc bike has less mileage than a 800cc car, which weighs more than the bike. How is it possible?
https://www.quora.com/An-800cc-bike-...e=eb3506e3HopeVehicle - Displacement - MaxPower@rpm - Top speed - weight - fuel economy
Kawasaki Z800 (bike) - 800cc - 112BHP @ 10200rpm - 230kmph - 230kg - 15kmpl
Maruti Alto 800 (car) - 800cc - 47BHP @ 6000rpm - 145kmph - 725kg - 20kmpl
It makes sense intuitively that even though the displacements (size - volume swept by the piston in the engine) are matched, the bike engine seems to deliver more 'performance', so it kinda seems reasonable that it'd use up more fuel.
BUT WHY?
Now the keyword here is the rpm. The rpm - revolutions per minute are proportional to the number of times combustion (controlled explosions in the engine using air and fuel) occurs in the engine per minute. This is where the fuel economy comes into play.
So, at 6000rpm there are 100 revs or crankshaft rotations which means 100/2 = 50 combustion events for a four stroke engine per second. At each one of these events an air-fuel mixture of the ratio ~15:1* is sucked in and burnt the engine.
Let's assume for now, approximately similar air-fuel mixtures are ratios in both engines. Taking this approximation, it's easier to see that for the same vehicle speed, if one engine is operating at higher engine speed, there'll be more combustion events per second, so it'll burn more fuel.
Most motorcycle engines are engineered (read optimized) to operate at higher rpms (the reason for that's for another answer). Now, since an Alto would on an average be made to operate at ~15-2500rpm, and the Kawasaki at ~5-8000rpm, you can get a rule of the thumb idea of their economies.
<Economy - 3 points to the car>
But what about the WEIGHT?
The Kawwi however has an ace up its sleeve. With one rider, it weighs almost a third of an Alto + driver. This added weight causes the Alto an inertial resistance, and also increases its tire friction. Also, given that power required at the wheel is tau*omega (torque into angular velocity), lower mass would necessitate lower torque, less throttle, and loosely less fuel.
<Economy - 3 points to the bike>
However there's another factor that contributes in the same way as above - AERODYNAMICS. What helps the car is that despite a bike being much more 'racetracky', a car would typically have a coefficient of drag (Cd) of around 0.35-0.5 (racecars ~ 0.2) while bikes would hover between 0.5 to 1.5 - the wide span being for a crouched rider on a faired Hayabusa to an upright rider on a Mahindra Mojo. This is very important at speeds above 80kmph.
<Economy - 1 point to the car>
______
FINAL
Overall, using the highly back-calculated points I gave for each element considered so far, it's 4 points to the car vs 3 points to the motorbike. So the Alto narrowly wins, as it does in real according to the figures I copied off the internet.
Please note however, that if it's a motorcycle engine which runs smoothly at super low rpms, and it's made to run at said rpms for the sole purpose of deriving the sick pleasure of winning this said contest, it is likely to annihilate the Alto as easily as it would in a race on a long dry racetrack.
PS: Hope this was helpful. I'd love to add illustrations and the like to make it look less like a wall of text but it ended up taking much longer than I thought it would already. Please feel free to point out/rectify any errata/exclusions/incorrect notions.
*Actually it varies from around 11:1 (rich) to 17:1 (lean), delivering more power at slightly richer than 14.7 (stoichiometric - neither rich nor lean - the air/fuel middle class) and more fuel economy at slightly leaner.
I just wrote an answer to An 800cc bike has less mileage than a 800cc car, which weighs more than the bike. How is it possible?
https://www.quora.com/An-800cc-bike-...e=eb3506e3HopeVehicle - Displacement - MaxPower@rpm - Top speed - weight - fuel economy
Kawasaki Z800 (bike) - 800cc - 112BHP @ 10200rpm - 230kmph - 230kg - 15kmpl
Maruti Alto 800 (car) - 800cc - 47BHP @ 6000rpm - 145kmph - 725kg - 20kmpl
It makes sense intuitively that even though the displacements (size - volume swept by the piston in the engine) are matched, the bike engine seems to deliver more 'performance', so it kinda seems reasonable that it'd use up more fuel.
BUT WHY?
Now the keyword here is the rpm. The rpm - revolutions per minute are proportional to the number of times combustion (controlled explosions in the engine using air and fuel) occurs in the engine per minute. This is where the fuel economy comes into play.
So, at 6000rpm there are 100 revs or crankshaft rotations which means 100/2 = 50 combustion events for a four stroke engine per second. At each one of these events an air-fuel mixture of the ratio ~15:1* is sucked in and burnt the engine.
Let's assume for now, approximately similar air-fuel mixtures are ratios in both engines. Taking this approximation, it's easier to see that for the same vehicle speed, if one engine is operating at higher engine speed, there'll be more combustion events per second, so it'll burn more fuel.
Most motorcycle engines are engineered (read optimized) to operate at higher rpms (the reason for that's for another answer). Now, since an Alto would on an average be made to operate at ~15-2500rpm, and the Kawasaki at ~5-8000rpm, you can get a rule of the thumb idea of their economies.
<Economy - 3 points to the car>
But what about the WEIGHT?
The Kawwi however has an ace up its sleeve. With one rider, it weighs almost a third of an Alto + driver. This added weight causes the Alto an inertial resistance, and also increases its tire friction. Also, given that power required at the wheel is tau*omega (torque into angular velocity), lower mass would necessitate lower torque, less throttle, and loosely less fuel.
<Economy - 3 points to the bike>
However there's another factor that contributes in the same way as above - AERODYNAMICS. What helps the car is that despite a bike being much more 'racetracky', a car would typically have a coefficient of drag (Cd) of around 0.35-0.5 (racecars ~ 0.2) while bikes would hover between 0.5 to 1.5 - the wide span being for a crouched rider on a faired Hayabusa to an upright rider on a Mahindra Mojo. This is very important at speeds above 80kmph.
<Economy - 1 point to the car>
______
FINAL
Overall, using the highly back-calculated points I gave for each element considered so far, it's 4 points to the car vs 3 points to the motorbike. So the Alto narrowly wins, as it does in real according to the figures I copied off the internet.
Please note however, that if it's a motorcycle engine which runs smoothly at super low rpms, and it's made to run at said rpms for the sole purpose of deriving the sick pleasure of winning this said contest, it is likely to annihilate the Alto as easily as it would in a race on a long dry racetrack.
PS: Hope this was helpful. I'd love to add illustrations and the like to make it look less like a wall of text but it ended up taking much longer than I thought it would already. Please feel free to point out/rectify any errata/exclusions/incorrect notions.
*Actually it varies from around 11:1 (rich) to 17:1 (lean), delivering more power at slightly richer than 14.7 (stoichiometric - neither rich nor lean - the air/fuel middle class) and more fuel economy at slightly leaner.
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