Coasting in neutral may feel cheaper but it uses more fuel.

mikewill34

Just read Martin's tips for fuel saving and I would like to clarify one of his points.

I have written parts of the software for the engine control unit (ECU) for some models of car and as is common with all other manufacturers we had a system called fuel cut off (FCO) which under certain conditions as its name suggests cut the fuel to the engine.

The conditions for the car I wrote the software for were speed greater than 10 mph and accelerator pedal at idle position and engine rpm greater than 1500.

These conditions exist when you take your door of the accelerator but leave the car in gear, hence using no fuel. But if you put the car in neutral you start to use fuel again once the rpm drops below 1500 rpm.

The figures above are for discussion only and may not be the same in all cars, but the principal is the same.

bazster

Coasting will cost a lot more than a few pence in fuel if PC Plod nabs you for not being in full control of your vehicle.

Ebe_Scrooge

bazster wrote: »

Coasting will cost a lot more than a few pence in fuel if PC Plod nabs you for not being in full control of your vehicle.

I know this is correct, but I've often wondered how on earth a plod could possibly know you're coasting in neutral.

HappyMJ

mikewill34 wrote: »

Just read Martin's tips for fuel saving and I would like to clarify one of his points.

I have written parts of the software for the engine control unit (ECU) for some models of car and as is common with all other manufacturers we had a system called fuel cut off (FCO) which under certain conditions as its name suggests cut the fuel to the engine.

The conditions for the car I wrote the software for were speed greater than 10 mph and accelerator pedal at idle position and engine rpm greater than 1500.

These conditions exist when you take your door of the accelerator but leave the car in gear, hence using no fuel. But if you put the car in neutral you start to use fuel again once the rpm drops below 1500 rpm.

The figures above are for discussion only and may not be the same in all cars, but the principal is the same.

What's your door doing on the accelerator?

The engine is either moved by burning fuel or by the drive wheels moving it hence not requiring any more fuel when in gear.

Not sure it makes any real difference. When in neutral the car slows down less than it would when in gear. You wouldn't need to accelerate as much when you need a bit more speed. Then we've got the debate about engine braking changing gears down when approaching a junction as opposed to physically using the brakes to slow down. Pennies...per trip.

bazster

Ebe_Scrooge wrote: »

I know this is correct, but I've often wondered how on earth a plod could possibly know you're coasting in neutral.

Smashing into a lamppost when your foot slips off the brake pedal might be a bit of a giveaway.

WellKnownSid

mikewill34 wrote: »

I have written parts of the software for the engine control unit (ECU) for some models of car and as is common with all other manufacturers we had a system called fuel cut off (FCO) which under certain conditions as its name suggests cut the fuel to the engine.

I remember building a circuit from the schematics in an electronics magazine in the 1980s which I attached to the overrun solenoid on the carburetor on my Ford Cortina. Those were the days!!

almillar

OK, with the caveat that the L test teaches you not to coast, because you're not in proper control of the vehicle, you can start to understand when it's OK to coast.
Down a hill, round corners - not OK. Floating up to a set of red lights, maybe OK.
mikewill34's assertion is wrong though that coasting costs more fuel absolutely. It'll only cost more if slowing down in gear (using engine braking) will stop you at the right time, as in, not too soon.
Assume we're approaching a red light, that you know will change.
The most efficient way is not to have to slow down at all. You've used energy accelerating that now goes to waste. So driving straight through at, say 20mph, with no acceleration or deceleration, is most efficient.
Next is to coast. Approaching at 30, clutch in, slow to, say, 10mph, light turns green, accelerate back up to 30. When coasting, you're using the same as sitting still with the engine running.
Next is to slow down in gear. The lower the gear, the more engine braking there will be, and the lower speed you can get down to. Whilst doing this, taking into account mikewill34's technical info above, you're using zero fuel. But from that same 30, you may slow down too much, light turns green, and you might have to accelerate more than above, to get back up to 30. In this case, coasting is better.
Next of course, is having to actually *shudder* touch the brakes! Of course this happens plenty in modern driving even with the best of anticipation. You should do this whilst staying in gear, so the engine helps with slowing down. Do it with the clutch in, and you'll just be changing brake pads more often.
So you're aiming for a steady speed everywhere basically and reducing acceleration and deceleration as much as possible.

Gloomendoom

bazster wrote: »

Coasting will cost a lot more than a few pence in fuel if PC Plod nabs you for not being in full control of your vehicle.

What does plod do if your car is fitted with a freewheel gearbox?

bazster

Gloomendoom wrote: »

What does plod do if your car is fitted with a freewheel gearbox?

If he thought you were not in full control of your vehicle I don't suppose he'd be remotely interested in clever-!!!!!! technical explanations as to why this might be the case.

facade

Gloomendoom wrote: »

What does plod do if your car is fitted with a freewheel gearbox?

Probably congratulate you on the outstanding condition of your 1959 Rover P4

I've not seen a free-wheel on anything since then.

Gloomendoom

bazster wrote: »

If he thought you were not in full control of your vehicle I don't suppose he'd be remotely interested in clever-!!!!!! technical explanations as to why this might be the case.

Or even if your car was in neutral or not.