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W_S · 12-11-2011, 06:56 PM

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!

12-11-2011, 06:56 PM	#1 (permalink)
W_S EcoModding Lurker Join Date: Jul 2011 Location: Poland Posts: 4 Thanks: 0 Thanked 0 Times in 0 Posts	Most fuel-efficient accelerating technique- diesel vs. petrol 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!