Car Qs slash myths

[cdfeto]

Is it true that ...

playing with/shifting gears in a car without the clutch (car in not started and not moving) will damage the clutch or/and gearbox?

engine braking (shifting down gears to slow down) consumes more petrol then normal braking because the revs go up? (although the gas pedal isnt pushed in, at all)

the ventilation (not A/C) uses a slight amount of petrol?


Please dont reply unless you can back your answers.

[red death]

Is it true that ...

playing with/shifting gears in a car without the clutch (car in not started and not moving) will damage the clutch or/and gearbox?

only very marginally - unless you're a totally mechanically unsympathetic gorilla.

engine braking (shifting down gears to slow down) consumes more petrol then normal braking because the revs go up? (although the gas pedal isnt pushed in, at all) - false, & unless you're running LPG, (or you live in USA) it's an acelerator pedal, or even a throttle

the ventilation (not A/C) uses a slight amount of petrol? - almost imeasurably. Ever noticed the revs drop when idling & you turn an electrical component on? More noticeable in rather old (pre 70's at least) cars with a generator rather than an alternator.

:)

[slip]

Is it true that ...

playing with/shifting gears in a car without the clutch (car in not started and not moving) will damage the clutch or/and gearbox?

If you have any mechanical experience at all, it will just feel wrong. Because it is. Clutch is safe really, gearbox and mainly shifting mechanisms are possible points of damage.

Well, with no clutch and assuming a normal car gearbox - you're going to be forcing 2 pieces of metal to align without much give at all. If they aren't perfectly aligned the force of pushing it into gear will have to create enough movement in one of the shafts connected (in the end) to the engine/crank or the diff/wheels for everything to line up and slot in. Don't see why on earth you would do it anyway. If you're going to do it, just use the clutch. Could bend a shifter fork if you were really forcing it.

engine braking (shifting down gears to slow down) consumes more petrol then normal braking because the revs go up? (although the gas pedal isnt pushed in, at all)

Depends on the car, some later models are programmed to be very fuel efficient under this sort of condition. Would be interesting to see if anyone has tuning values for idle vs 4000rpm 0% throttle opening. Even if they were the same, the RPM would cause more fuel consumption. A carburetted vehicle would definitely use more fuel.

the ventilation (not A/C) uses a slight amount of petrol?

Well, it doesn't consume petrol per se. It's just an electrically powered internal car fan, and is not parasitic like A/C. It will create more load on the electrical side of things, but wether this is enough to cause any drop in a well maintained later model car I doubt. (A/C is physically turned by the engine via a belt, and is therefore parasitic - and will increase fuel consumption if the same driving style is to be maintained)

[Binaural]

Removed post to go attend remedial post-reading class :(

[sawtell]

slip summed it up super nicly.

[scblack]

Well, it doesn't consume petrol per se. It's just an electrically powered internal car fan, and is not parasitic like A/C. It will create more load on the electrical side of things, but wether this is enough to cause any drop in a well maintained later model car I doubt. (A/C is physically turned by the engine via a belt, and is therefore parasitic - and will increase fuel consumption if the same driving style is to be maintained)
Agree with all your answers. Note also that some new cars have electric air-con systems, so they do not need the belt from the engine to run.

As far as fuel consumption goes that would be the same as a ventilation system.

[morto1980]

Is it true that ...
engine braking (shifting down gears to slow down) consumes more petrol then normal braking because the revs go up? (although the gas pedal isnt pushed in, at all)


As others have said it depends on the sophistication of the engine. I know that my mid-nineties Mazda has a throttle position sensor that cuts / reduces fuel to the engine if the revs are rising out of sync with the throttle position - saving fuel in this sort of situation. However, as one of our car club members pointed out - it is better to slow the car with brakes which are designed to slow the car & are cheaply replaceable rather than your drivetrain (diffs, CV joints, clutch, gearbox, engine) which isn't.

Another thought is that things like ABS and EBD are not activated when using engine braking - although I guess if you're slowing the car dramatically enough with engine braking that these systems would activate you'd have bigger problems than a loss of control.

[Vicious_Fishes]

Depends on the car, some later models are programmed to be very fuel efficient under this sort of condition. Would be interesting to see if anyone has tuning values for idle vs 4000rpm 0% throttle opening. Even if they were the same, the RPM would cause more fuel consumption. A carburetted vehicle would definitely use more fuel.



the ecu turns the injectors off on my suby when it's under 4500rpm (closed loop). when i have no throttle, i really have no throttle ;p

[Breaka]

When engine breaking I'll always double declutch (it feels weird if I don't, like turning/changing lanes without indicating). There's no doubt that this doesn't use more fuel as you have to adjust the revs manually via the accelerator.

[Daver]

Depends on the car, some later models are programmed to be very fuel efficient under this sort of condition. Would be interesting to see if anyone has tuning values for idle vs 4000rpm 0% throttle opening. Even if they were the same, the RPM would cause more fuel consumption. A carburetted vehicle would definitely use more fuel.

Surely engine braking is going to use more fuel than coasting to a stop. Obviously to get the engine to brake for you at some point you need to change gears. When you change gears, you need to blip it otherwise it just jumps into gear, and the car hops as well.

I just tried it again thisarvo, according to the computer in the car, if I'm off the throttle, at whatever the revs, with the car in gear, it's telling me that I'm not using any fuel...

[john92]

i know in landcruiser diesals when the air con is turn on the rev up slighly at idle

[sprocket]

i watched myth busters about a month ago and they tested the air-con thing and found that the car used more fuel with the air-con on then driving with the windows down

[24alpha]

i watched myth busters about a month ago and they tested the air-con thing and found that the car used more fuel with the air-con on then driving with the windows down

On a hot day, i don't car if it does, I'm using my A/C!:D

Thanks for your posts guys! Good questions and good answers. I've learnt something today!:)

[jjperko]

Surely engine braking is going to use more fuel than coasting to a stop. Obviously to get the engine to brake for you at some point you need to change gears. When you change gears, you need to blip it otherwise it just jumps into gear, and the car hops as well.

I just tried it again thisarvo, according to the computer in the car, if I'm off the throttle, at whatever the revs, with the car in gear, it's telling me that I'm not using any fuel...

I'm thinking that for example when you try and roll start a car or moto without any fuel in it or when it has ignition problems the momentum of the car down the hill keeps the wheels rolling and therefore engine turning over. The engine is still running though when you engine brake so that confuses me. All cars with those computers I've driven tell me 0.0 as well.

[NH_]

i watched myth busters about a month ago and they tested the air-con thing and found that the car used more fuel with the air-con on then driving with the windows down

until they hit about 55mph and then the wind drag caught up and made it better to use air con

or maybe it was 35mph, it was something like that i cant remember

[ajay]

Surely engine braking is going to use more fuel than coasting to a stop. Obviously to get the engine to brake for you at some point you need to change gears. When you change gears, you need to blip it otherwise it just jumps into gear, and the car hops as well.

I just tried it again thisarvo, according to the computer in the car, if I'm off the throttle, at whatever the revs, with the car in gear, it's telling me that I'm not using any fuel...

AFAIK, the fuel consumption meters just measure throttle posistion, now considering you dont need to use any throttle to change down, it would naturally not move.

[red death]

..snip...A carburetted vehicle would definitely use more fuel....

are you saying fuel is literally sucked through carb ports despite a closed butterfly?

(setting aside gear changing blips as mentioned in another post)

[Tomas]

the ecu turns the injectors off on my suby when it's under 4500rpm (closed loop). when i have no throttle, i really have no throttle ;p

Hey, so does mine :).

[Ivan]

I'm thinking that for example when you try and roll start a car or moto without any fuel in it or when it has ignition problems the momentum of the car down the hill keeps the wheels rolling and therefore engine turning over. The engine is still running though when you engine brake so that confuses me. All cars with those computers tell me 0.0 as well.

No, my computer still tells me how much fuel I'm using when I'm off the throttle, it even tells me how much i'm using when I'm parked and Idleing.

From memory It uses around 2L/hr when parked and idling with the AC on.

[Vicious_Fishes]

ok seriously, engine braking in a nutshell.


if the engine was burning fuel, it wouldn't be braking the car as the wheels wouldn't have any resistance to movement from the friction/etc of turning the engine. if it was burning fuel then there wouldn't be any resistance to the forward motion of the car (i.e the energy it takes to turn the engine) & it wouldn't slow down. ;)

[Ivan]

ok seriously, engine braking in a nutshell.


if the engine was burning fuel, it wouldn't be braking the car as the wheels wouldn't have any resistance to movement from the friction/etc of turning the engine. if it was burning fuel then there wouldn't be any resistance to the forward motion of the car (i.e the energy it takes to turn the engine) & it wouldn't slow down. ;)

Ok, so why did honda develop a MotoGP motor that would specifically stop injecting fuel under braking to save fuel?

My uneducated opinion is that when you are at 0% throttle ( engine braking, idleing, whatever) there is still a minimum amount of fuel being injected into the cylinder on every cycle.

Ducati have resurrected their "tickover clutch", tested and discarded at Jerez in 2005, which basically disengages the clutch under braking, leaving the bike with no engine braking at all. The advantage to this design is the significant reduction in fuel consumption under braking, but the disadvantage comes when you let of the brakes. The engine has to fire up instantly to a speed matching the bike speed, otherwise the tire chirps and the rider gets spat off. I know it a shit source but I'm trying
http://blogger.xs4all.nl/daisy/articles/189145.aspx

[Elbo]

I always engine brake, and only use the brakes for stopping in traffic or going down a steep hill, or when I think people need to see my brake lights go on. I figure that when using no throttle, it's not injecting any more fuel than it needs to, to keep running, therefore the same fuel consumption at idling as with engine braking.

Not sure on wear and tear though. Whenever we go 4WDing, if it's a steep hill you just chuck it in gear and let it go, but that is more to do with keeping traction rather than fuel consumption.

I think in newer cars it's not worth it engine braking, because changing gears is so smooth and easy, that you may as well just go from 4th to 2nd and then stop and change to 1st, whereas in my '85 4Runner, I go from 4th to 3rd to 2nd and back to first and crawl along. I have a fairly low gear ratio so I can pretty much just let the clutch out in 1st and crawl along without stalling at traffic lights. On turns I will slow enough to change from 4th to 3rd and then change to 2nd just before entering the corner, which slows it right down and then just roll through the corner.

This is my understanding of engine braking, may not work for others, because I'm the type that slows down 100m before the traffic lights and takes about to same to get back up to 40km/h

[BM Epic]

One thing i do know,you cannot hurt the clutch unless you are using it,you can hurt it with the motor running or not,but with clutch not being used and changing gears while the car is not running,you can't hurt it!

[slip]

are you saying fuel is literally sucked through carb ports despite a closed butterfly?

(setting aside gear changing blips as mentioned in another post)

EFI cars do it (depending on car/computer as previously discussed) and a carby is mechanically controlled partly by throttle, but also by vacuum. My knowledge is rusty on them as it's been a long time since working with carbys. If it is going to suck fuel at idle with minimal vacuum, then it's going to suck fuel at 4000rpm with more vacuum.

[Vicious_Fishes]

Ok, so why did honda develop a MotoGP motor that would specifically stop injecting fuel under braking to save fuel?

what i have quoted from you is exactly what i was saying - the ecu turns the injectors off but keeps the spark so the plug tip stays hot. it's just basic physics, there has to be resistance to the motion of the wheels in order for engine braking to work, and it comes in the form of the energy used to rotate the motor under the leverage of whatever gear you're in. if fuel was going in there it wouldn't have any resistance would it ? the advantage of engine brake + hydraulic brake means more braking force (provided your brakes can't reach the max grip levels of your tyres but, we'll leave that out) and you can stop faster.

motorbikes i have nfi about, i don't know the levels of computer management, etc. motogp and any form of racing is basically the opposite to daily driving btw.

[Vicious_Fishes]

pfft, carburettors :rolleyes:

[Gripper]

what i have quoted from you is exactly what i was saying - the ecu turns the injectors off but keeps the spark so the plug tip stays hot. it's just basic physics, there has to be resistance to the motion of the wheels in order for engine braking to work, and it comes in the form of the energy used to rotate the motor under the leverage of whatever gear you're in. if fuel was going in there it wouldn't have any resistance would it ? the advantage of engine brake + hydraulic brake means more braking force (provided your brakes can't reach the max grip levels of your tyres but, we'll leave that out) and you can stop faster.

motorbikes i have nfi about, i don't know the levels of computer management, etc. motogp and any form of racing is basically the opposite to daily driving btw.

the above EFI feature has been around ever since its introduction, it's known as "fuel cut on over run" and is part the reason why EFI saves fuel and EFI motors are lasting longer as there is minimal "bore wash" from the excess unburnt fuel getting past the piston rings and washing the piston/bore and dilluting the oil in the motors sump.

[Ivan]

what i have quoted from you is exactly what i was saying - the ecu turns the injectors off but keeps the spark so the plug tip stays hot. it's just basic physics, there has to be resistance to the motion of the wheels in order for engine braking to work, and it comes in the form of the energy used to rotate the motor under the leverage of whatever gear you're in. if fuel was going in there it wouldn't have any resistance would it ? the advantage of engine brake + hydraulic brake means more braking force (provided your brakes can't reach the max grip levels of your tyres but, we'll leave that out) and you can stop faster.

motorbikes i have nfi about, i don't know the levels of computer management, etc. motogp and any form of racing is basically the opposite to daily driving btw.

Doesn't the resistance caused by engine barking come from the compression of gases within the chamber? thats why 2 strokes have F@ck all engine braking?

Ever had a car conk out on you? then you know what engine braking feels like when there's no combustion in the chamber. I just have to think that there is still fuel entering the cylinders of 99% of cars under engine braking.

[bebbinator]

well the best thing to do is to not try them anyway because it really doesnt matter

[Vicious_Fishes]

Doesn't the resistance caused by engine barking come from the compression of gases within the chamber? thats why 2 strokes have F@ck all engine braking?

Ever had a car conk out on you? then you know what engine braking feels like when there's no combustion in the chamber. I just have to think that there is still fuel entering the cylinders of 99% of cars under engine braking.

so, what are you trying to argue exactly ?

i'm not sure on how to explain your theory with an engine conking out at say, 60 km/h, as if there really was no fuel then simple engine braking should give you the same resistance, in physical theory ? anyone else help out here ? all i can say is that i know the ecu turns the injectors off, there really is no fuel going in, so a physical explanation from someone that knows more than me would be awesome :)

at any rate, even if there was fuel going into the engine, provided that it isnt any more than idle levels, you'd still be in front because you'd use the same amount of fuel as braking with the clutch in, but wear out your brakes less and be significantly safer.

[red death]

...at any rate, even if there was fuel going into the engine, provided that it isn't any more than idle levels, you'd still be in front because you'd use the same amount of fuel as braking with the clutch in...

I agree. This is just what I would have thought, no-one could argue there would be NO fuel going in, only that it'd be no more than if there was no pressure on the accelerator pedal so reducing fuel input to idle capacity.

[Vicious_Fishes]

but i do know for a fact ecu's will turn off the injectors completely when you take your foot of the throttle, my subaru will actually drop below redline before the ecu detects that it's about to stall and throws some fuel in. it takes maybe 1/3 of a second from dropping to chugging a little as the injectors turn back on to idling fine again.

[Angry Atheist]

It may help to know how manual transmission actually works (http://auto.howstuffworks.com/transmission1.htm). Notice that the splined gear wheels are always in mesh and you are moving a large toothed plate instead?

[Ivan]

at any rate, even if there was fuel going into the engine, provided that it isnt any more than idle levels, you'd still be in front because you'd use the same amount of fuel as braking with the clutch in, but wear out your brakes less and be significantly safer.

No, thats not correct, because under engine braking the RPM is significantly higher than at idle, so fuel (at minimum levels) would be injected a lot more often.

But, after reading around the net, some people say that on newish cars the ECU turns off all fuel, so as far as new cars go I'm probably wrong. It would be nice to hear from a car mechanic or something. There is an automotive engineer on these forums that's being suspiciously quiet...... ;)

[3viltoast3r]

Its more fuel efficient to use the A/C is your speed is greater than 70km/h an hour. Otherwise any speed LESS than 70km/h its more fuel efficient to have the windows down. Seen on mythbusters, although that was on on of those stupidly huge big american 'mummy drives kid to school' cars......4wd's which have nether been off road, for use in urban/city. I hate people who have 4wd's for on-road purposes only.