Extreme smoothness - how to smooth overrun zero fuel?

[Somewhatfoolish]

The jerkiest thing in my normal driving has for a long time been where the fuel injection goes from some fuel to no fuel when one zeroes the throttle. You can get around the jerk here by always having some throttle on but with my fairly recent obsession with hypermiling when I'm not doing anything else I try to be driving with zero fuel fairly often, basically on all downhills. And my primary car doesn't go to zero fuel consumption until about 2000rpm so I can't really slip the clutch or anything. Naturally I can rev match if I'm in a higher gear and get into the lower gear fairly smoothly, so it's not changing into gear that's the problem - but if you let off what seems to minimum throttle at 3000rpm there is almost always a small jerk.

Any cunning techniques to make this jerk not noticeable? I'm thinking I may be able to come up with an idiosyncratic technique involving the brake at the same time but this should be the kind of thing that has been encountered and solved by chauffeurs and limousine drivers. But then again it doesn't really happen on automatic transmissions and before autos became commonplace engines had carburetters, so maybe not?

[BertBert]

I've never noticed this kind of jerk before.
Bert

[VinceFox]

Says he's been here for 49 months, apparently.

[S. Gonzales Esq.]

But seriously: are you jumping off the throttle, or releasing it gently over a second or so?

[BertBert]

VinceFox said:

Says he's been here for 49 months, apparently.

that's not very polite!

[Dave^]

VinceFox said:

Says he's been here for 49 months, apparently.

[davepoth]

I'd check your engine mounts.

[james_gt3rs]

davepoth said:

I'd check your engine mounts.

I think most cars do it to some degree don't they? I've noticed my car does it less if I put in Shell V Power, so OP could try that?

[sparkybean]

Somewhatfoolish said:

The jerkiest thing in my normal driving has for a long time been where the fuel injection goes from some fuel to no fuel when one zeroes the throttle. You can get around the jerk here by always having some throttle on but with my fairly recent obsession with hypermiling when I'm not doing anything else I try to be driving with zero fuel fairly often, basically on all downhills. And my primary car doesn't go to zero fuel consumption until about 2000rpm so I can't really slip the clutch or anything. Naturally I can rev match if I'm in a higher gear and get into the lower gear fairly smoothly, so it's not changing into gear that's the problem - but if you let off what seems to minimum throttle at 3000rpm there is almost always a small jerk.

Any cunning techniques to make this jerk not noticeable? I'm thinking I may be able to come up with an idiosyncratic technique involving the brake at the same time but this should be the kind of thing that has been encountered and solved by chauffeurs and limousine drivers. But then again it doesn't really happen on automatic transmissions and before autos became commonplace engines had carburetters, so maybe not?

I have noticed this, and yes, it is what you are describing. All cars have it, but some have it worse than others depending on how they are mapped. I remember driving a 2 year old i10 that had it bad enough that i kept the power on for pretty much the rest of the drive. My MX5 has very little of this cutoff 'jolt' so i guess mazda were thinking about it when they made the car. Though the sort of people who drive MX5's are more likely to be in tune with the finer points of driving than i10 drivers, i guess (nothing against i10 drivers )

Another problem similar to this is when the engine starts injecting fuel again when the revs fall below the decel fuel cutoff, but i drive around this in most cases.

The only way i can think to get around the jolt you are describing is to let off the throttle as much as you can without the ECU cutting the fuel until you are below a threshold rpm. But as you said you are hypermiling, so the only away to make the car even smoother (and we are talking silky smooth, perhaps imperceptable to some at this point) is to start tinkering with things. Upping your EGR% at low loads would be easiest (i use the term easy loosely here). If tinkering with cars is your bag of course.

Edit: Of course it could also be as simple as your engine mounts/diff mounts, as mentioned above.

[davepoth]

sparkybean said:

I have noticed this, and yes, it is what you are describing. All cars have it, but some have it worse than others depending on how they are mapped. I remember driving a 2 year old i10 that had it bad enough that i kept the power on for pretty much the rest of the drive. My MX5 has very little of this cutoff 'jolt' so i guess mazda were thinking about it when they made the car. Though the sort of people who drive MX5's are more likely to be in tune with the finer points of driving than i10 drivers, i guess (nothing against i10 drivers )

Down to the engine mounting in the main. If you think about it, the MX-5, with the engine inline, has a long base fore and aft, and is fixed at the gearbox as well. The "shunt" isn't strong enough to lift the gearbox (it's at the end of a long lever and the forces are attempting to lift from near the fulcrum), so the effect is minimised. On a transverse engined car the lever is much shorter, so the force is acting much nearer to the end of the lever, hence a lot more shunt.

[Hooli]

VinceFox said:

Says he's been here for 49 months, apparently.

[Somewhatfoolish]

Fifty now

agree that there is also a jolt when engine starts injecting fuel again but it does seem easier to drive with that in a way because you can make the jerk part of the acceleration - i.e. the jerk is basically the 2nd derivative of speed so you can just about make it part of that by increasing acceleration through the jerk. You can't really do that with deceleration through engine alone, but just typing this out has made me realise perhaps the way to do it is to blend with the brake, something I will experiment with going forward.

Anyone denying this effect occurs, take your car up to 4500 rpm in 1st, stabalise it at whatever speed that is, then let off the throttle. In 99% of manuals there is a noticeable jerk however you do it.

[abbotsmike]

In first and second gear the engine has a whole lot more leverage on the rest of the drivetrain though, so it's hardly surprising! In higher gears then it's much less noticeable.

[Fazt]

davepoth said:

Down to the engine mounting in the main. If you think about it, the MX-5, with the engine inline, has a long base fore and aft, and is fixed at the gearbox as well. The "shunt" isn't strong enough to lift the gearbox (it's at the end of a long lever and the forces are attempting to lift from near the fulcrum), so the effect is minimised. On a transverse engined car the lever is much shorter, so the force is acting much nearer to the end of the lever, hence a lot more shunt.

Dave, surely the transverse engine moves from front to back as described, but the inline engine, wouldn't lift the gearbox, it would attempt to twist the box by moving left and right hence the reason this would feel smoother.

in all fairness I find that this jerk can be removed from the drive by practicing smooth control, I don't have an issue in any of the vehicles i drive. or maybe i've just been lucky

[Gary C]

Somewhatfoolish said:

The jerkiest thing in my normal driving has for a long time been where the fuel injection goes from some fuel to no fuel when one zeroes the throttle. You can get around the jerk here by always having some throttle on but with my fairly recent obsession with hypermiling when I'm not doing anything else I try to be driving with zero fuel fairly often, basically on all downhills. And my primary car doesn't go to zero fuel consumption until about 2000rpm so I can't really slip the clutch or anything. Naturally I can rev match if I'm in a higher gear and get into the lower gear fairly smoothly, so it's not changing into gear that's the problem - but if you let off what seems to minimum throttle at 3000rpm there is almost always a small jerk.

Any cunning techniques to make this jerk not noticeable? I'm thinking I may be able to come up with an idiosyncratic technique involving the brake at the same time but this should be the kind of thing that has been encountered and solved by chauffeurs and limousine drivers. But then again it doesn't really happen on automatic transmissions and before autos became commonplace engines had carburetters, so maybe not?

Not many cars cut fuelling anymore.

[BertBert]

Gary C said:

Not many cars cut fuelling anymore.

The majority of ecu controlled cars cut fuel on the overrun, which is what I assume the OP is talking about.
Bert

[Fazt]

BertBert said:

The majority of ecu controlled cars cut fuel on the overrun, which is what I assume the OP is talking about.
Bert

agree with Bert most of the cars cut fuel when free wheeling.

when free wheeling down the ramp in my local shopping centre, my injectors turn off and the car is completely silent until the point i get to the bottom of the ramp and you hear (but don't feel) the injectors return to operating.

it sounds like the engine is off and then turned back on

[PhillipM]

davepoth said:

I'd check your engine mounts.

This.

[davepoth]

Fazt said:

Dave, surely the transverse engine moves from front to back as described, but the inline engine, wouldn't lift the gearbox, it would attempt to twist the box by moving left and right hence the reason this would feel smoother.

in all fairness I find that this jerk can be removed from the drive by practicing smooth control, I don't have an issue in any of the vehicles i drive. or maybe i've just been lucky

Thinking about it there are probably two forces at work in driveline shunt - the inertia of the engine as it moves about on the mounts a bit when you change acceleration, and the turning forces on the engine as you've said. On a transverse jobbie those will all be working together to shunt the engine forwards when letting off, but they work perpendicular to each other on an inline layout with the inertia being mitigated by the length of the heavy bit of the drivetrain.

[Fazt]

inline or transverse, the engine wants to twist off its mounts due to the fact that it is giving a turning drive. if there were no mounts resisting the turning force of the engine. it would not be able to overcome the resistance of the drive train and the weight of the vehicle.

there will be no difference in the pick and let off feeling in the car. other than the direction that the engine rocks. all will feel the same though.

if there is a jerk between engine breaking and then transitioning back to the throttle, then surely there is play/slack in the driveline?

i have a Landrover that is used for off roading and there is a quarter turn of play in the centre diff this gives a jolt between engine breaking and throttle, but this is fairly easy to overcome.