A few questions regarding fuel consumption

Chickabiddybex

Inspired by another thread, when you're at the lights if you sit with your foot on the clutch (and in 1st) instead of in neutral why is that bad?
How does it affect fuel consumption?

If you're rolling down a hill with the clutch down instead of in gear why is that bad?
How does it affect fuel consumption?

Is it better to have your foot down all the way in 4th or just a bit in 3rd? (Going up a hill for example)

If it makes a difference my car is 16 years old. (Endura e)

Bongles

Chickabiddybex wrote: »

Inspired by another thread, when you're at the lights if you sit with your foot on the clutch (and in 1st) instead of in neutral why is that bad?

Because the clutch release mechanism is under load so arguably you are contributing to its wear and eventual demise.

Chickabiddybex wrote: »

How does it affect fuel consumption?

There's no difference between sitting in neutral and sitting in gear with the clutch down.

Chickabiddybex wrote: »

If you're rolling down a hill with the clutch down instead of in gear why is that bad?

As with any coasting, you are further away from being able to drive the car with the throttle than if you were in gear. Arguably that's only bad if you didn't assess, before deciding to coast, that you won't need that drive for the time being. Downhill you have only your brakes to control your speed so you will need to work them harder (increased wear) than if you had engine braking working for you and in extremis you might suffer brake fade (although I think you'd have to try quite hard).

Chickabiddybex wrote: »

How does it affect fuel consumption?

Coasting down hill will need more fuel because with the drive disengaged the momentum of the vehicle can't keep the engine turning over so that has to be achieved by burning fuel instead.

Chickabiddybex wrote: »

Is it better to have your foot down all the way in 4th or just a bit in 3rd? (Going up a hill for example)

If you're going to hold a constant throttle position for a period of time, you're likely to do better in a gear that allows you to use a smaller amount of throttle travel - so 3rd with the pedal just a bit down in your example.

HappyMJ

Chickabiddybex wrote: »

Inspired by another thread, when you're at the lights if you sit with your foot on the clutch (and in 1st) instead of in neutral why is that bad?
How does it affect fuel consumption?

If you're rolling down a hill with the clutch down instead of in gear why is that bad?
How does it affect fuel consumption?

Is it better to have your foot down all the way in 4th or just a bit in 3rd? (Going up a hill for example)

If it makes a difference my car is 16 years old. (Endura e)

At a junction...zero difference. You should put it in neutral and handbrake for safety reasons. If you are hit with force from behind (losing your feet off the pedals and maybe knocking your head on something) and the car is in gear then the car will move off into a dangerous position in the intersection...if you are in neutral with the handbrake on the car will move a bit but will stop again without moving into the intersection too far.

Rolling up to the junction if you are in gear then no fuel is used. The car moving will keep the engine running. If you put the car in neutral whilst rolling up to the junction then fuel needs to be used to keep the engine turning over.

Going up a hill you probably really should be in 3rd if necessary then if you need to put your foot down then you have some power in reserve without having to change down...losing momentum in the process.

Ultrasonic

Chickabiddybex wrote: »

Is it better to have your foot down all the way in 4th or just a bit in 3rd? (Going up a hill for example)

If you have to floor it to get up the hill in 4th then you are better off in 3rd. But if the car is happy in both gears, then generally 4th would be better, since engines are more efficient at higher load. (For a more detailed explanation have a look here:

http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps#Volkswagen_Jetta_TDI_1.9L_ALH_1999.5-2003)

Ultrasonic

HappyMJ wrote: »

Rolling up to the junction if you are in gear then no fuel is used. The car moving will keep the engine running. If you put the car in neutral whilst rolling up to the junction then fuel needs to be used to keep the engine turning over.

That is all true, but if circumstances allow you to roll up to the junction in neutral without braking this will be the more fuel efficient way to do so - see the freewheeling thread. Normally you need to slow down more quickly and so doing so in gear is the way to go.

colino

"if circumstances allow you to roll up to the junction in neutral without braking this will be the more fuel efficient way to do so" No it isn't. Keeping it in gear, foot off the gas, means the wheels are driving the gearbox which turns the engine so no fuel required to be injected. Putting it in neutral means the car has to maintain base idle itself and injects fuel to maintain those revs.

westwood68

Just seared for BSFC and came up with this on Wikipedia:-

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption#Examples_of_values_of_BSFC_for_shaft_engines

Energy Efficiency:-

Prius petrol engine = 37% (still higher than most petrol engines)
Junkers Jumo 204 turbocharged two-stroke diesel (from 1931!) = 39.8%
Volkswagen 3.3 V8 TDI (from 2000) = 41.1%
Audi 2.5 litre TDI (from 1990) = 42.5%

I suppose the first 2 are cheating because there is no way a petrol can match a diesel in efficiency but so much for progress between the last 2?

WTFH

I think comparing a 3.3V8 with a 2.5 (V6 from memory) is probably not a straight comparison. Also, you might want to look at a car from 2010 to compare back rather than just the 10 year gap between 1990 and 2000

Autocar wrote:

In 1980 the typical petrol engine was 23 per cent efficient and a diesel 28 per cent efficient. Now, those figures have improved by approximately half.

westwood68

WTFH wrote: »

I think comparing a 3.3V8 with a 2.5 (V6 from memory) is probably not a straight comparison. Also, you might want to look at a car from 2010 to compare back rather than just the 10 year gap between 1990 and 2000

The older engine question was an in-line 5 and it may not be a fair comparison of layouts, but the comparison to Indirect Injection on the older engine and Common Rail on the new one is interesting.

Seems that Common Rail really just helps with driveabilty and power delivery rather than making a big difference to efficiency.

WTFH

Yes, I don't think that common rail is noticeably more efficient, but the computers controlling the overall system make a difference.

Bongles

colino wrote: »

"if circumstances allow you to roll up to the junction in neutral without braking this will be the more fuel efficient way to do so" No it isn't. Keeping it in gear, foot off the gas, means the wheels are driving the gearbox which turns the engine so no fuel required to be injected. Putting it in neutral means the car has to maintain base idle itself and injects fuel to maintain those revs.

Note that Ultrasonic said if circumstances allow you to roll up to the junction in neutral without braking. That bit's important. Personally I think it's a big 'if' but others might disagree.

Imagine circumstances do allow that. Picture the point on the road at which Ultrasonic drops into neutral and starts to coast. Now picture yourself driving the same road (let's assume it's flat to keep things simple). Under engine braking you will decelerate more than under coasting. You therefore cannot start your engine braking as early as Ultrasonic starts to coast otherwise you won't make it to the junction. You have to stay on the gas beyond that point before lifting off.

Once you've lifted off you will be saving fuel compared to Ultrasonic's coasting, but up until that point you were losing out by having to stay on the gas longer. The argument is that overall you lose out because the latter outweighs the former.