Most fuel-efficient accelerating technique- diesel vs. petrol

[W_S]

Hello

I have searched for this topic on the forum but did not find a clear answer. Most sources on ecodriving, and also many posters here, say that the most economical way to accelerate is to use high engine load (say, 75%) but to shift early. I have often come across suggestions to shift up at around 2000 rpm in diesels and around 2500 rpm in petrol-powered engines.

I understand that the basis for it is BSFC, which is usually the lowest around maximum torque (is it?). In modern diesels such an explanation sounds logical, as they usually have maximum torque around 1500-2000 rpm. However, most (non-turbicharged) petrol engines generate maximum torque around 4000 rpm. So shouldn't they have best BSFC around 4000 rpm? If so, shouldn't the driver use revs around 4000 while accelerating to get the best economy?

I suppose a reason for the suggestion to shift at 2500 rpm is that maximum torque is achieved at (let's say) 4000 rpm at full load only, while at 75% load the peak may be at lower rpms. Is this correct?

One more question: how about turbocharged petrol engines such as VW's TFSI? They have maximum torque at quite low revs. What is the most fuel-efficient behaviour in engines with such characteristics?

Many thanks in advance for clraifying this!

[JRMichler]

That's something that you have to find for yourself. You need a trip MPG readout, whether one built in to the vehicle, or a Scangauge or Ultragauge. Pick a route that you drive regularly - I used my route home from work. The trip MPG starts at zero, then check it at the same location every time. Try different techniques - accelerate hard, accelerate like a granny, etc. Keep in mind that you don't want to get in other people's way.

My truck does best when I step on it and shift at 2500 RPM.

[W_S]

Hi

I am not interested in my own car's response to various techniques but in a general rule that could be applied to all (internal combustion-powered) vehicles.

So any feedback of a more general nature would be much appreciated.

Many thanks

[BHarvey]

75-80% load in my car means 1.6 gph, which is BARELY accelerating even in the low gears.

To have any useful acceleration I have to get to at least 2.0 gph on flat ground.

[99metro]

Generally, IMO, is that gas/petrol engines should be accelerated at just below "open loop", which is right around 75% throttle. You want to keep the oxygen sensor in the fuel control loop - closed loop. This is for open country where there is distance between stops. Stop and go traffic should be easy throttle between stop lights. When the gasoline engine has to work hard, like accelerating, generally the least fuel used is a peak torque.

Diesels are another story. It will take some practice to see what works best. Generally, one ought to shift around 1200 rpm, then shift at around 1700 for the last gear. This is my opinion, not hard and true statement for everything.

[Piwoslaw]

Originally Posted by W_S

most (non-turbicharged) petrol engines generate maximum torque around 4000 rpm. So shouldn't they have best BSFC around 4000 rpm? If so, shouldn't the driver use revs around 4000 while accelerating to get the best economy?

I suppose a reason for the suggestion to shift at 2500 rpm is that maximum torque is achieved at (let's say) 4000 rpm at full load only, while at 75% load the peak may be at lower rpms. Is this correct?

My guess (correct me please!) is that at 4000rpm there is a lot more internal friction to overcome than at 2500, so as engine speed crawls up towards max torque efficiency increases but so does friction (at a faster rate). At some point one starts to overcome the other and that's when you should shift.

[Sularus]

what is the best marker for determining acceleration rate. I have tried utilizing both GPH and keeping it below 2.25 gph, and I have also used % engine load. I am not sure how accurate the load portion of my Ultra Gauge is. I have not found there is a specific difference. I am trying to figure this out for both regular accleration from stoping and for the portion of my commute I am able to do a mile P&G.

[JRMichler]

Originally Posted by W_S

Hi

I am not interested in my own car's response to various techniques but in a general rule that could be applied to all (internal combustion-powered) vehicles.

So any feedback of a more general nature would be much appreciated.

Many thanks

Is no general rule. You need to find this for each vehicle because it's not as simple as staying the zone of best BSFC.

For example, part of the energy to accelerate is spent spinning up the rotating mass of the engine. That energy is greatest in the lowest gears, so the optimal shift points will be at lower RPM in the lowest gears.