Driving Economically.

Joe_Horner

The point (7 in the list) about accelerating at full throttle is right provided you do it properly.

Any petrol engine is at it's most efficient when it's running at the peak of it's torque curve with a fully open throttle. That's just a matter of the physics and even Scotty couldn't find a way round it!

Obviously most cars can't accelerate while staying at the peak of their torque curve because they only have certain fixed drive ratios to choose from so, if you have 5 gears, there are only 5 speeds possible with those engine revs.

But in practice, most cars will be close to maximum torque anywhere between about 2 and 4k rpm and accelerating with full throttle, while using the gears to keep in that range, will give the best efficiency. Or get a well set-up CVT and plant your foot every time

Full throttle and screaming up to the redline on each change obviously won't

mikey72

Max torque on mine is at 3800 rpm, max power is 5600 rpm.
I don't hold it on full throttle to overtake, if I can avoid it, as that also injects the most petrol.
That badly affects the mpg.

It's better in mine to gently build the speed up, and plan to overtake when you don't need to accelerate hard.
Same as going from a 30 to 70 limit. I don't need to accelerate quickly from one speed to the higher. It works better to keep it at around 3800rpm and let it do the work there.

Urban for my car is 23 or 24 mpg, I normally achieve 28 or so fully loaded.

Strider590

Joe_Horner wrote: »

Any petrol engine is at it's most efficient when it's running at the peak of it's torque curve with a fully open throttle. That's just a matter of the physics and even Scotty couldn't find a way round it!

I'm sorry, but for the benefit of others I have to point out that peak torque is not at full throttle and the best way to achieve full power and torque is progressive application of right foot, NOT stamping on the pedal.

New cars are more forgiving, the ECU tries to smooth out the throttle input, but in an older car (or something modified) you'd just flood the engine and cause it to stumble.

Joe_Horner

Strider590 wrote: »

I'm sorry, but for the benefit of others I have to point out that peak torque is not at full throttle and the best way to achieve full power and torque is progressive application of right foot, NOT stamping on the pedal.

New cars are more forgiving, the ECU tries to smooth out the throttle input, but in an older car (or something modified) you'd just flood the engine and cause it to stumble.

With respect, the bolded bit is a nonsense statement. You also can't obtain "full power and torque" at the same time

"Peak torque" may or may not be at full throttle depending on the conditions. You simply can't compare the two because the peak torque speed is an engine speed, and the speed an engine happens to run at depends on load and gearing to the drive wheels as well as throttle opening.

However, the maximum efficiency of any engine operating (approximately) on the Otto cycle will always be at whatever engine speed produces maximum torque for that particular engine and with a wide-open throttle.

That's nothing to do with the ECU, it's a simple fact of the basic principles involved:

The thermal efficiency for an engine is highest at the peak of it's torque curve and the volumetric efficiency is highest at full throttle. So, when the two conditions coincide - peak torque RPM and full-open throttle, the engine is at it's most efficient overall.

You can't maintain that condition while accelerating with a conventional transmission because the engine speed will increase as you go faster, until you change to the next gear. But you can get very close by using short changes and keeping the revs around the torque peak while having the throttle fully open to maximise volumetric efficiency.

If you doubt that, consider what a clogged air filter does to fuel consumption, and that's a far smaller restriction to airflow than a closed throttle

Delivery_Driver

Joe_Horner wrote: »

The point (7 in the list) about accelerating at full throttle is right provided you do it properly.

Any petrol engine is at it's most efficient when it's running at the peak of it's torque curve with a fully open throttle. That's just a matter of the physics and even Scotty couldn't find a way round it!

Obviously most cars can't accelerate while staying at the peak of their torque curve because they only have certain fixed drive ratios to choose from so, if you have 5 gears, there are only 5 speeds possible with those engine revs.

But in practice, most cars will be close to maximum torque anywhere between about 2 and 4k rpm and accelerating with full throttle, while using the gears to keep in that range, will give the best efficiency. Or get a well set-up CVT and plant your foot every time




Full throttle and screaming up to the redline on each change obviously won't

Can't argue with physics , but thats assuming perfect conditions ie noone in front of you, straight road where you don't have to slow down again any time soon.Very rarely applicable to roads these days.

Joe_Horner

Delivery_Driver wrote: »

Can't argue with physics , but thats assuming perfect conditions ie noone in front of you, straight road where you don't have to slow down again any time soon.Very rarely applicable to roads these days.

True But it gives an idea why "brisk" acceleration when it's needed is better than taking 5 minutes to creep up to 60. Although, in my Daf, that is wide open throttle :rotfl:

passatrider

I think you need to get out more..;)

mikey72

Whilst not disputing "ye cannae change the laws of physics"

Maximum torque for my car is 3800rpm.
it does near enough exactly 20mph per 1000rpm, so 76mph is of the top of the torque band.
5600rpm is maximum power, so 112mph.

I get much better economy at 56mph, or 2800rpm, neither full throttle, or close to the torque or power band.

Why?

Joe_Horner

mikey72 wrote: »

Whilst not disputing "ye cannae change the laws of physics"

Maximum torque for my car is 3800rpm.
it does near enough exactly 20mph per 1000rpm, so 76mph is of the top of the torque band.
5600rpm is maximum power, so 112mph.

I get much better economy at 56mph, or 2800rpm, neither full throttle, or close to the torque or power band.

Why?

Because overall fuel economy depends on the whole vehicle, including aerodynamic drag, which increases exponentially with speed (and mechanical friction which has quite complex variations with speed, but let's keep it simple).

If peak torque at 3800 rpm is at 76mph in top then 56mph is 2800 rpm, where most "road" engines will still be producing close to max torque - maybe 90 -95% of it if you get it on a rolling road. So by dropping from 76MPH to 56MPH you'll have maybe a 5 or 10% drop in engine efficiency.

But your speed is only 0.74 of what it was, so the drag at 56 will only be about 54% of what it was at 76. Seeing as the aerodynamic drag is by far the biggest "use" of fuel at those speeds, you swap a 10% reduction in the efficiency of burning your fuel for a 46% reduction in the fuel you need to burn to overcome the drag.

mikey72

Kinda destroys the good economy at the speed limit theory then.