Variable Valve Timing and Fuel Economy

[HustleRussell]

My old BMW 525i is doing bang on 3,000RPM at 70mph.

Above 3,000RPM, VANOS starts to advance the inlet valve timing.

Now, I've been driving at 70mph in the belief that the engine will be more economical with the retarded valve timing.

I'm wrong, aren't I?

I have always noticed that this car seems to be less economical when I try to drive it economically (keeping engine speed down by using higher gears / larger throttle openings).

[stevieturbo]

HustleRussell said:

I have always noticed that this car seems to be less economical when I try to drive it economically (keeping engine speed down by using higher gears / larger throttle openings).

large throttle openings will never be an economical way to drive, so of course it will use more fuel.

[anonymous-user]

stevieturbo said:

HustleRussell said:

large throttle openings will never be an economical way to drive, so of course it will use more fuel.

er, not really true tbh!



You need to understand there are two facets to maximising fuel economy.


The first, and really most important point is to minimise your road load. ie, to drive the car in such a fashion as to require the least power to push it along.

The obvious way is to minimise braking. Heat lost to your brakes can never be recovered and is immediately "wasted".

Then you have Drag. Rolling friction (tyres, gears, bearing etc) and Aerodynamic losses. Rolling friction is pretty much directly proportional to vehicle speed, and aero drag is proportional to the square of vehicle speed. So, the slower you go, the less power you need to produce (and hence the less fuel you need to burn). Below about 50mph, rolling friction dominates, above that speed aero drag rapidly takes over.




Once you have minimised you roadload, then it becomes a case of how to operate the car to maximise the efficiency at which that power is generated. And here we start to get trade offs with the basic premise of minimum road load.

For example, to minimise engine friction you need to "downspeed" the engine as much as possible (ie, drive in the highest gear possible), but below a certain road speed, you can't use top gear. At some low speed (probably aroud 30 mph, you start to "waste" more energy in engine friction than you save by driving slower.

The there is the complex effects of engines Brake Specific Fuel Consumption. Things like ignition timing, throttling losses and tuning etc mean that an engine makes it's power output at differing efficiency's as it's load and speed change. Modern engines are highly efficient, but only in a small operating zone (hence 8 spd gearboxes!) and keeping the engine in that narrow efficient zone means you make the roadload power at the highest efficiency, and hence use the least fuel.




So, i summary, you need to:

Never Brake!

Drive at the lowest speed at which you can use Top gear

If travelling downhill or slowing to a halt, select neutral (to downspeed engine the max amount) and coast

Change up as soon as possible, aim to never use more than about 1500rpm
Use approx 60% throttle openings when accelerating (enough to avoid throttling losses, but not result in the engine going into a "high load" operating zone (overfuelling + retarded ignition)


In reality, modern busy traffic conditions make most of those^^ pretty difficult to do tbh

[227bhp]

Max_Torque said:

stevieturbo said:

HustleRussell said:

large throttle openings will never be an economical way to drive, so of course it will use more fuel.

er, not really true tbh!



You need to understand there are two facets to maximising fuel economy.


The first, and really most important point is to minimise your road load. ie, to drive the car in such a fashion as to require the least power to push it along.

The obvious way is to minimise braking. Heat lost to your brakes can never be recovered and is immediately "wasted".

Then you have Drag. Rolling friction (tyres, gears, bearing etc) and Aerodynamic losses. Rolling friction is pretty much directly proportional to vehicle speed, and aero drag is proportional to the square of vehicle speed. So, the slower you go, the less power you need to produce (and hence the less fuel you need to burn). Below about 50mph, rolling friction dominates, above that speed aero drag rapidly takes over.




Once you have minimised you roadload, then it becomes a case of how to operate the car to maximise the efficiency at which that power is generated. And here we start to get trade offs with the basic premise of minimum road load.

For example, to minimise engine friction you need to "downspeed" the engine as much as possible (ie, drive in the highest gear possible), but below a certain road speed, you can't use top gear. At some low speed (probably aroud 30 mph, you start to "waste" more energy in engine friction than you save by driving slower.

The there is the complex effects of engines Brake Specific Fuel Consumption. Things like ignition timing, throttling losses and tuning etc mean that an engine makes it's power output at differing efficiency's as it's load and speed change. Modern engines are highly efficient, but only in a small operating zone (hence 8 spd gearboxes!) and keeping the engine in that narrow efficient zone means you make the roadload power at the highest efficiency, and hence use the least fuel.




So, i summary, you need to:

Never Brake!

Drive at the lowest speed at which you can use Top gear

If travelling downhill or slowing to a halt, select neutral (to downspeed engine the max amount) and coast

Change up as soon as possible, aim to never use more than about 1500rpm
Use approx 60% throttle openings when accelerating (enough to avoid throttling losses, but not result in the engine going into a "high load" operating zone (overfuelling + retarded ignition)


In reality, modern busy traffic conditions make most of those^^ pretty difficult to do tbh

That was rather a lot of time & effort spent not answering the question at all, in any way.

[andyiley]

Max_Torque said:

So, i summary, you need to:

If travelling downhill or slowing to a halt, select neutral (to downspeed engine the max amount) and coast

Er.....NO!

If you select neutral your modern fuel injected engine is using fuel to keep itself running, and yes I know it isn't much.

In the right (read highest suitable) gear for the speed you are travelling at, and no throttle opening there is no fuel going into the engine, it doesn't need it, as the engine is being turned over by the wheels turning through the gearbox and the throttle position sensor is telling the ECU this, so it shuts down the injectors.

[stevieturbo]

andyiley said:

Er.....NO!

If you select neutral your modern fuel injected engine is using fuel to keep itself running, and yes I know it isn't much.

In the right (read highest suitable) gear for the speed you are travelling at, and no throttle opening there is no fuel going into the engine, it doesn't need it, as the engine is being turned over by the wheels turning through the gearbox and the throttle position sensor is telling the ECU this, so it shuts down the injectors.

And having the car in this manner will decelerate the car unless it's a steep hill, so you'd need to use more fuel again to recoup that speed.

So some hills will be better in neutral, others better in-gear and on the overrun.

But really if you're concerned to those extremes, you have mental issues.

[anonymous-user]

andyiley said:

Er.....NO!

If you select neutral your modern fuel injected engine is using fuel to keep itself running, and yes I know it isn't much.

In the right (read highest suitable) gear for the speed you are travelling at, and no throttle opening there is no fuel going into the engine, it doesn't need it, as the engine is being turned over by the wheels turning through the gearbox and the throttle position sensor is telling the ECU this, so it shuts down the injectors.

er, Yes! if you don't want to come to a stop, then you need to reduce parastic losses to a minimum. Having the engine being turned by the wheels is a MAJOR source of energy wasteage, much more so than the tiny amount of fuel being consumed to keep the engine idling.

[HustleRussell]

Some interesting stuff here but it doesn't really answer my specific question which is will the engine be operating more efficiently off VANOS or on it? Peak torque is up at 4,200RPM which suggests to me the engine would be more efficient after. VANOS has advanced the inlet cam timing.

I'm not a hypermiler, you won't see me coasting in neutral or labouring the engine at below 1,500RPM any time soon.

[anonymous-user]

HustleRussell said:

Some interesting stuff here but it doesn't really answer my specific question which is will the engine be operating more efficiently off VANOS or on it? Peak torque is up at 4,200RPM which suggests to me the engine would be more efficient after. VANOS has advanced the inlet cam timing.

I'm not a hypermiler, you won't see me coasting in neutral or labouring the engine at below 1,500RPM any time soon.

Efficiency doesn't come into it. If you're reving at 4200rpm you have so much engine friction that the couple of percent reduction in throttling losses due to Vanos is irrelevant!

(Also worth noting that the Camshaft position will optimised to reduce volumetric efficiency at part load, so as to reduce those throttling losses)

[Mr2Mike]

HustleRussell said:

Some interesting stuff here but it doesn't really answer my specific question which is will the engine be operating more efficiently off VANOS or on it? Peak torque is up at 4,200RPM which suggests to me the engine would be more efficient after. VANOS has advanced the inlet cam timing.

I'm not a hypermiler, you won't see me coasting in neutral or labouring the engine at below 1,500RPM any time soon.

Peak torque only infers the point of highest volumetric efficiency, not overall efficiency. Higher RPM implies:

1) Higher frictional losses
2) Smaller throttle opening for a given power, increasing pumping losses and lowering dynamic CR (hurts thermal efficiency).

It's a fallacy that engines are more efficient at small throttle openings, even the hypermiling people worked this out a long time ago with their "pulse and glide" silliness. Engine downsizing in modern cars means you will be putting the engine under more load for any given power requirement, which means it's operating in a more efficient area then a heavily throttled large displacement engine.

In summary, it's more than likely close to optimal where it is. If you have a fuel computer, why not check the difference between 4th and 5th on a few miles of flat motorway.

Edited by Mr2Mike on Monday 29th June 19:23