[mark]

mark asked the Naked Scientists:

I am a keen motorcyclist and it occurred to me that whilst my motorbike only has an engine capacity of 600cc, it develops approximately 110bhp - a similar amount to many family cars with far bigger engines, typically 1800cc to 2000cc.

I can quite easily achieve 50 mpg and it also produces roughly half the carbon emissions of a car with the same power.

My question therefore is - why can't we fit motorbike engines to cars? Not only would we achieve more miles per gallon but there would be far less emissions for every one of those miles. Everyone would benefit - it would be cheaper to run it terms of buying the fuel in the first place and it would be hugely beneficial to the environment.

Am I being too cynical to suggest only the government would lose out on tax revenue from petrol sales?

Many thanks for a great show, Mark.

What do you think?

[lyner]

Your motorcycle engine doesn't consume as much fuel mainly because it is not doing as much  work in any given hour. It may be developing a lot of hp during brief bursts of acceleration but it is very seldom developing anything like its maximum for any extended time. The mass and drag are both a lot less than for a car and cruising at, say, 80mph just takes less energy.
I don't know whether motorcycle engines are designed to work for 100,000 miles either, as car engines do. What is their expected running life?

[lightarrow]

mark asked the Naked Scientists:

I am a keen motorcyclist and it occurred to me that whilst my motorbike only has an engine capacity of 600cc, it develops approximately 110bhp - a similar amount to many family cars with far bigger engines, typically 1800cc to 2000cc.

I can quite easily achieve 50 mpg and it also produces roughly half the carbon emissions of a car with the same power.

My question therefore is - why can't we fit motorbike engines to cars? Not only would we achieve more miles per gallon but there would be far less emissions for every one of those miles. Everyone would benefit - it would be cheaper to run it terms of buying the fuel in the first place and it would be hugely beneficial to the environment.

Am I being too cynical to suggest only the government would lose out on tax revenue from petrol sales?

Many thanks for a great show, Mark.

What do you think?

The main reason motorcycles power/cc ratio is higher than that of cars is due to torque. Motorcycles' engines have a too low torque to push a heavier car; this is due to the lower excursion of the piston (hope to have translated well); for the same reason, motorcycles' engines run much faster (12.000 - 16.000 rpm).
All this means that, to run a car with a motorcycle's engine, you should use much "shorter" (even here, hope to have trans. well) gears; at the end, it would be like to run a car always in short gears (1° - 2° - 3°) and so the consumptions would be higher.

[lyner]

The main reason motorcycles power/cc ratio is higher than that of cars is due to torque. Motorcycles' engines have a too low torque to push a heavier car; this is due to the lower excursion of the piston (hope to have translated well); for the same reason, motorcycles' engines run much faster (12.000 - 16.000 rpm).

This confuses me - not for the first time- because I have heard it said before.

For a given capacity and all other things being equal, all engines would be expected to produce the same power. The engine would have a torque proportional to the area (the piston area times the pressure) times the crank length (which is half the piston movement). As the engine gets 'shorter', the area gets bigger inversely proportional to the stroke and the crank length increases in proportion. The torque - i.e. the product - would be the same, would it not?

A shorter stroking engine can rev higher because the velocity of the piston is less for a given swept volume (although the piston is more massive) but it is harder to keep the compression ratio as high because of the combustion chamber volume. So 'all things' aren't really equal.

Furthermore, whatever the torque, if the gearing is appropriate, ( I think you may mean 'lower' gears, Lightarrow) you can drive the wheels optimally and the final performance should really depend upon Power (motive Force times speed). This goes against received wisdom but I don't see what's wrong with my argument.
I bet the real answer is a lot more complicated. Any ideas?

[lightarrow]

The main reason motorcycles power/cc ratio is higher than that of cars is due to torque. Motorcycles' engines have a too low torque to push a heavier car; this is due to the lower excursion of the piston (hope to have translated well); for the same reason, motorcycles' engines run much faster (12.000 - 16.000 rpm).

This confuses me - not for the first time- because I have heard it said before.

For a given capacity and all other things being equal, all engines would be expected to produce the same power. The engine would have a torque proportional to the area (the piston area times the pressure) times the crank length (which is half the piston movement). As the engine gets 'shorter', the area gets bigger inversely proportional to the stroke and the crank length increases in proportion. The torque - i.e. the product - would be the same, would it not?

That's right. I don't know the answer; I suspect it also depends on the "steeper" torque curve of a motorcycle, which gives it a "decent" value of torque at high rpm only; in a motorcycle this is less a problem because it's lighter and with less aerodynamic resistance with respect to a car.

[lyner]

I bet it's to do with expected lifetime. Most motorcycles are driven relatively frantically for fewer hours/miles per year. It's very seldom that the engine in a car falls apart. People would never buy the brand again.
It always amazes me that motorcyclists are quite prepared to use as many gears as you care to give them. Car drivers limit themselves to 5 - and that is after about a hundred years of motoring. One of my first vehicles had only three.

[techmind]

mark asked the Naked Scientists:

I am a keen motorcyclist and it occurred to me that whilst my motorbike only has an engine capacity of 600cc, it develops approximately 110bhp - a similar amount to many family cars with far bigger engines, typically 1800cc to 2000cc.

I can quite easily achieve 50 mpg and it also produces roughly half the carbon emissions of a car with the same power.

You can't consider the engine in isolation. A car has a mass of 1 to 1.5 tonnes (more if its a chuncky 4x4). A motorbike -I'm guessing- has a mass more like 300kg. All that extra "deadweight" in a car takes energy to accelerate (you dump the energy as heat in the brakes when you stop) or to haul up a hill. The car also has greater aerodynamic cross-section, so probably greater air-resistance when travelling at speed.

While I suspect there is some truth in the arguments that motorbike engines may be able to produce a greater peak-to-sustained power ratio than a car engine, I reckon the difference in total vehicle mass (and size) is the real issue.
The suggestion that a motorbike has more gears (news to me!) may also indicate that a motorbike engine can be used at closer to its optimum (most efficient) torque for more of the time???

[lyner]

Six speed boxes are not uncommon in motorbikes (road bikes) - you don't find many road cars with more than four or five.
I think the guy who goes down our road at 2a.m. has about ten; it sounds like it to me as he shatters the night air.

I am sure that the main reason for more mpg on a bike is the simple fact that it just doesn't do as much work.

"Energy in = work done + wasted energy" always applies.

[DonBrown]

I had a car powered by a 250cc Villiers 2 stroke motorbike engine. I got around 40-50mpg at at time when small cars got around 30mpg, but I'd guess a 250cc motorbike would have got around 100mpg. My car (Bond Mk.G)was smaller than most small cars (though I'd say larger than the BMW Isetta or Messerschmidt KR200 and similar "bubble" cars, as it had 4 seats.) My top speed was around 45mph and the poor petrol consumption was due to the fact (as suggested in previous posts) that one drove with one's foot to the floor much of the time (ie. full throttle.) 
The body was constructed mainly of aluminium and fibreglass, reinforced with fairly thin "U" section steel braces. Unfortunately the strong vibration of a single cylinder 2 stroke engine caused fatigue cracks in the aluminium at stress concentration points. (So perhaps the car manufacturers who stuck with cheap pressed steel, had more than economy and planned obsolescence in mind.)

But nostalgia makes me digress.  The fact is, high power ≠ high efficiency, and low cylinder capacity does not necessarily imply low fuel consumption (Gordon Brown please note!) if it has to work much harder.
I think it is also the case that 2 strokes could get more power per cc because they have a power stroke every rev, rather once every two revs with a 4 stroke.  Their power output still depends on the petrol input, so you can only get this extra power out if you can get extra petrol in (and exhaust gasses out.) It is the difficulty in achieving this at varying engine speeds, that is the 2 stroke's problem.  If you can solve this, then lower capacity can mean smaller size and less weight, which would give a more efficient engine.
Similarly, Diesels are intrinsically more efficient because they have a higher compression ratio, but because of the higher pressures, they need to be stronger, hence heavier, lowering efficiency.

And a bit of a btw.
Quote:"I can quite easily achieve 50 mpg and it also produces roughly half the carbon emissions of a car with the same power."
Surely the mpg exactly predicts the carbon emissions.  Where else does the carbon come from? If you get 50mpg, presumably the comparable car must get 25mpg, if it has double the carbon emission.
It puzzles me why the exchequer makes such complicated rules (and requires expensive testing) to put extra tax onto high emission cars, when simply taxing fuel would exactly penalise carbon emissions.

[lyner]

It puzzles me why the exchequer makes such complicated rules (and requires expensive testing) to put extra tax onto high emission cars, when simply taxing fuel would exactly penalise carbon emissions.

Too damned simple, I think!

[daveshorts]

I think there are some issues with the way people asses costs themselves - they rarely add up all the trips to a petrol station they are going to do with a car, but the price of the car and the amount of road tax you are going to be paying is a lot more obvious.

I don't know if the government have the incentives right, but they have to do slightly odd things with the tax system as people are not rational agents - note to all the economists out there.

[techmind]

It puzzles me why the exchequer makes such complicated rules (and requires expensive testing) to put extra tax onto high emission cars, when simply taxing fuel would exactly penalise carbon emissions.

It creates jobs, which apparently is "a good thing". The fact that all these jobs are unneccessary and therefore all the associated resource- and energy-use is completely 100% wasteful is overlooked.

Let's face it, if an engineer ran the country we'd lose 90% of the jobs calculating, collecting and paying tax and benefits overnight ;-).


The show-room could, next to the official mpg figures, publish an annual petrol cost (at current prices) based upon the mpg. Bt that'd be too simple too.

[syhprum]

Compression ratio is rather a misnomer what is relevant is the expansion ratio how much energy is obtained from the burning fuel before it is exhausted