Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Most fuel-efficient accelerating technique- diesel vs. petrol

polarelec · 08-10-2014, 06:29 AM

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.

08-10-2014, 06:29 AM	#9 (permalink)
polarelec EcoModding Lurker Join Date: Aug 2014 Location: France Posts: 2 Thanks: 0 Thanked 1 Time in 1 Post	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.