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Old 12-11-2011, 06:56 PM   #1 (permalink)
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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!

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Old 12-11-2011, 08:02 PM   #2 (permalink)
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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.
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Old 12-13-2011, 03:02 PM   #3 (permalink)
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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
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Old 12-13-2011, 05:13 PM   #4 (permalink)
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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.
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Old 12-14-2011, 05:27 PM   #5 (permalink)
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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.
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Old 12-15-2011, 01:47 AM   #6 (permalink)
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Quote:
Originally Posted by W_S View Post
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.
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Old 12-21-2011, 09:13 PM   #7 (permalink)
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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.
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Old 12-22-2011, 01:20 PM   #8 (permalink)
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Quote:
Originally Posted by W_S View Post
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.
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Old 08-10-2014, 06:29 AM   #9 (permalink)
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Hello. The original question asks for clarification of a subject rarely discussed (it seems). Quite so! To know where one is in the labyrinth of choices about it, a short recapitulation of the theory is useful looking at it from an unusual way, then all compromises can be "seen".

The efficient burn of the fuel to produce energy may be at engine maximum torque which is also a certain rotational speed (usually). At that moment the multiple of unique torque times unique speed gives a specific power output which is also unique. Then most were to prefer to connect that engine at that fixed and economic performance to a continuously variable transmission and the questionner is right to suggest it's going to sound high revving. Such a vehicle will accelerate and move against wind resistance with a fixed power. Someone content to accelerate taking a very long time and finally going very slowly against the air (for the same vehicle size, tyre friction etcetera) will arrive more economically and apparently late. The problem is that to make it efficient, he or she is going to need a small engine deployed only at max torque and when he reaches a significant hill - the vehicle won't work.

The reality is else than a technique of changing gears - one really needs to change out the motor. I know we're educated to suppose the gearbox takes care of the problem, yet then, as the questionner remarks . . . we're rarely or never using the engine at it's most efficient point - becaaaaaaaause . . . . they're all over-powered (to be ready and capable of going up hill). You think this mad? It is! If you accelerate away from the stoplights with a typical 4000 rpm torque and efficiency (which it is), by changing gears, you'll notice you've slammed up behind the car in front. Everyone, but everyone is obliged to accelerate slowly, well out of the efficient range to stay on the road and behind the slowest and most efficient. When I mention "slowest" one is careful with the term, because they may accelerate slowly yet have a very high speed once cruising (airplanes and trains come to mind).

Trains do it well . . . really. They pick a schedule, choose an engine for a specific cruise speed on the flat just able to overcome friction and airdrag . . . . then they accept (tolerate) whatever acceleration will result when leaving the stations . . . . then when they meet an incline - they "change out the motor" to more correctly stress the exploitable torque . . . i.e. they add more locomotives as just and only necessary, then unhitch them at once. Any other idea is inefficient, wasteful and due to lack of patience with the hitching and unhitching.

There are canny people who have really understood this point and there are engines which can shut down a few cylinders to put the remaining pistons to their maximum torque, yet the high revving noise is unpopular.
Wikipedia explains "Variable deplacement" engines rather well. Take a V8 then only run one half or make the combustion chambers variable in size. Do the customers care enough, that much about economy? Apparently not as each regular vehicle manufacturer offers something that "people" turn down.

Unlike a train it's less convenient to drop off the excessive motorisation from a car once an hour - physically. Rockets do it and it is, really crucial to have fuel economy in their field, in stages as the speed/acceleration requirement reduces. Some planes can do something similar; some pilots of three engined jet airliners flew on only two (which seems an "obvious" economy - aren't I just silly?), however those two are going to be working "hard" at higher thrust (not maximum, yet not far off). Those two will burn more fuel per hour yet more efficiently while they do, just beating a three running engine configuration overall.

I hope those aware of this 'story' will tolerate my oversimplification and notice that I attempt to answer the question for clarification rather than ignore the topic of gearshift. Most people simply can't shift up at efficient rpm using their overpowered cars (ready for hillclimbs) without slamming into the cars in front or ripping off rubber on their own tyres.

good day, hope you enjoyed reading.
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Old 08-11-2014, 07:39 AM   #10 (permalink)
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Maximum torque point being most efficient is nonsense that became widely believed because it happens to be almost true for some engines and most people don't understand how engines work. In reality, economy engines have a very limited rev range, so the maximum torque being in the 2-4k range and the maximum efficiency being in that range as well is just a convenient coincidence of there not being that wide a range of operating speeds in the first place.

You have to realize torque is a product of efficiency (thermal, the one we care about) and volumetric efficiency. Take in a lot of air and burn it inefficiently, you could still end up with more torque than taking in a lot less air but burning it more efficiently.

It's hard to tell exactly where you're going to be most efficient, but generally it's at relatively high loads and a reasonable rpm. If the engine is spinning too slowly then the heat has more time to escape, if it's spinning too fast friction is eating up too much power. As far as load goes, there's a big "fixed cost" of friction at any given rpm (by that I mean, there is a large component of friction that you can think of as being load-independent), and most gasoline engines are controlled by a throttle plate which wastes energy, so you want to be producing as much power as possible all else being equal.

However not all else is equal; modern engines are clever and can squeeze extra torque out by running rich (worse for efficiency, not all engines do this), and adjusting valve timing and such (could possibly reduce efficiency), so you don't want 100% of the available power. Also, at higher loads, more uncaptured energy leaves the exhaust. That's how you get the 75-80% number.

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