nick 3 Posted July 3, 2006 (edited) I understand how transmission loses work but I was just wondering why it is measured as a percentage of the power output? If, for instance, a gearbox takes 3 bhp to overcome the friction inside it, what does it matter if there is 100bhp giong though it or 1000bhp. Shouldn't it still take 3bhp to overcome the friction? If the loses are given as a percentage (say 10% to make the sums easy!) the the 100bhp engine will lose 10bhp through the gearbox, but the 1000bhp engine will lose 100bhp through the same gearbox (this is all theoretical before someone says that the same gearbox won't be able to cope!) I'm not really fussed about the answer, just curious. Nick Edited July 3, 2006 by nick Share this post Link to post Share on other sites
number2301 1 1 Cars Posted July 3, 2006 Surely its more likely that instead of the gearing taking 3 hp to run it that it loses power in heat and things like that? Share this post Link to post Share on other sites
jonnie205 1 Posted July 3, 2006 Transmission losses is not realy a percentage of engine power its a drag or bhp figure that will remain consatnt as long as the transmission stays the same. If you fit a 200bhp engine or a 50 bhp one the losses will still be the same. Often more powerfull cars have bigger losses due to bigger gears, diffs, shafts and bearings etc to cope with the higher power. Share this post Link to post Share on other sites
Sandy 191 Posted July 3, 2006 Transmission loss (in the gearbox) varies with load and speed, it's partly constant and partially increasing with load and speed, so it's not a linear relationship. Imagine you're dragging a box along the floor, then put it kid in the box and try dragging it, the effort required increases, then try doing it faster, the effort increases again and so on. The gearbox is only part of the loss though, the tyres are a much bigger issue than people often realise. When an engine is coasting odwn after a power run on the rollers, the tyres barely deflect, but when the engine is pulling hard, the tyres deflect alot more and generate significant heat as a result. That heat is lost power that you won't see on coast down, or even on the road for that matter, since the flat surface affects the tyre deformation differently to a roller's radius. Share this post Link to post Share on other sites
nick 3 Posted July 3, 2006 But isn't the rule of thumb that everyone uses 15% transmission loss? Nick Share this post Link to post Share on other sites
Ryan 99 Posted July 3, 2006 But isn't the rule of thumb that everyone uses 15% transmission loss? Nick And it's just that - a rule of thumb. Using a percentage value instead of a fixed value is usually good enough for preactical purposes, unless you fancy running computer simulations of the gearbox internals. Share this post Link to post Share on other sites
nick 3 Posted July 3, 2006 Don't get hung up on the g/box thing, I was using that as an example part of the whole transmission (including tyres, driveshafts etc...) I was just wondering why it is based on a % . I'll assumed that the "industry" need a standard and that is how it is done. I realise it would be totally impractical (if not impossible!) to work out transmission losses for every individual engine/transmission combination. Nick Transmission losses is not realy a percentage of engine power its a drag or bhp figure that will remain consatnt as long as the transmission stays the same. If you fit a 200bhp engine or a 50 bhp one the losses will still be the same. Often more powerfull cars have bigger losses due to bigger gears, diffs, shafts and bearings etc to cope with the higher power. Thats what I'm saying, but people always say "minus 15% (or whatever) transmission loss" Nick Share this post Link to post Share on other sites
Batfink 201 Posted July 3, 2006 Transmission loss (in the gearbox) varies with load and speed, it's partly constant and partially increasing with load and speed, so it's not a linear relationship. Imagine you're dragging a box along the floor, then put it kid in the box and try dragging it, the effort required increases, then try doing it faster, the effort increases again and so on. The gearbox is only part of the loss though, the tyres are a much bigger issue than people often realise. When an engine is coasting odwn after a power run on the rollers, the tyres barely deflect, but when the engine is pulling hard, the tyres deflect alot more and generate significant heat as a result. That heat is lost power that you won't see on coast down, or even on the road for that matter, since the flat surface affects the tyre deformation differently to a roller's radius. It goes further than that. The frictional force acting between road and tyre increases with speed too, not just with tyre deformation Share this post Link to post Share on other sites
Mandic 0 Posted July 25, 2006 http://www.dynamometer.fsnet.co.uk/power-l...lywheel-BHP.htm Cheers Ziga Share this post Link to post Share on other sites
