Naked Science Forum
General Science => General Science => Topic started by: mikegreenland on 16/01/2019 13:02:39
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My car is capable of driving at 40mph in gears 2,3,4,5,6. What is the effect on fuel economy in each gear. In simple physical terms on a flat and level surface with a constant wind whatever gear I use will need the fuel energy to propel 1500kg at 40mph. Does the increase in rpm by using a lower gear cause other inefficiencies?
I think I should make it clearer that I am talking of a constant speed of 40mph not accelerating to 40mph.
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If you look at the power/speed and torque/speed curves of your engine, you will see that both have a peak. The peaks are flatter and closer together for a diesel than for a petrol engine, and the falloff at high RPM is sharper for a diesel.
You need maximum torque to accelerate from rest, so the lowest (takeoff) gear for a petrol engine may be lower than for the equivalent diesel. The power required to cruise at any given speed increases with speed, so the highest (cruising) gears of a petrol engine require more RPM per MPH than a diesel and the high-RPM falloff of a diesel necessitates many more intermediate gears in a truck, so that you can take off at 1000 RPM (max torque) and cruise at 60 mph at 1200 RPM (max power). For an equivalent petrol engine the max torque may be at 3000 RPM and max power at 5000. Problem is that you will be burning a lot of fuel at 5000 RPM, because efficiency decreases with speed, so car manufacturers add a couple more gears so you can cruise at 2000 - 2500 RPM in 6th, but will probably have to change down into the max torque region to overtake.
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What is the effect on fuel economy in each gear.
You seem to be more interested in efficiency than in max torque or power. To a point, it is more fuel efficient in the higher gears due to less speed of moving parts and hence less friction. But put it in too high of a gear and all the energy might be required keeping it from stalling, requiring more fuel to add more torque to keep the car at speed. In addition, low RPM under load will damage the piston seals and lead to early wear on the engine.
I have a 2-liter and 40 MPH (about 1600 RPM) seems the low end of where I can cruise on level ground without any acceleration. Any more power and I need to downshift. A 1500 lb car probably has less than a 2-liter, so I wouldn't run it much under 1800 RPM.
My car stupidly tries to give me hints on the dashboard, and I totally ignore that since it has no sense at all. Hitting a steep hill at 55, I find myself losing speed in 5th and need to drop to 4 or less, but the dash is telling me to upshift to 6. An automatic transmission is much better at knowing what gear to be in since it is the thing doing the choosing.
I get better mileage with my car (a no-frills standard) than does a typical hybrid driver, but I use a lot more tricks than just using the correct gear. A hybrid vehicle is great for city driving, but not so good out in the hills.
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The efficiency and power of petrochemical engines as are standard, as is stated above by others, contrasts markedly to the electric engine of electric vehicles, which have no lag and no optimum rpm just constant power and acceleration.
https://simanaitissays.com/2013/07/20/tranny-talk/
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I also had the brief experience of driving a DAF variomatic, this little box of a car of my Grandfather's.
The interesting effect is that engine RPM is a function of how hard you press the gas pedal, and not particularly a function of your speed at all. I actually saw one parked next to me a few months ago here in the USA. Never seen one here before that.
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In the bad old days we used to have a manifold vacuum gauge that effectively indicated efficiency: a wide-open throttle or low RPM reduces the pressure differential across the throttle butterfly, so max vac at any given speed indicates minimum fuel consumption. Whilst this was usually achieved in the highest gear with a 3 or 4 speed box, it often turns out to be in 4th gear at 40 mph with a 6-speed box: in any higher gear the engine will be turning on the low-speed side of the power and torque curves.
Modern cars with 6 speed boxes tend to have multifunction display panels, one function of which is "instantaneous MPG". Worth investigating the max MPG at various constant speeds on a runway or a flat motorway section (preferably at 3 a.m. if you don't want trucks hooting at you). Police instructors generally recommend 3rd, 4th or 5th in any restricted area, saying that the economy of a modern fuel injected car in the 30 - 50 mph range is not improved by using a higher gear and speed control is better in the lower gears.