What to are the factors that affect acceleration?
10-22-2012, 03:16 PM
#1
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Mercedes s350
What are the factors that affect acceleration?
Hey guys,
I've always had a hard time figuring out what I should be looking at in a car to
predict acceleration. It's obvious that more power and less weight will yield
higher acceleration, but should I be looking at torque or horsepower?
I've read that more horsepower=higher top speed and more torque means
reaching that top speed faster, but what difference does it make if the
manufacturers limit the max speed to say 260 KPH?
I've also noticed that two cars with the same 0-60 time can have a hugely
different 0-100 or 0-150 acceleration time.
How about accelerating on a steep gradient? If I understood correctly from
what I've read, then you'll need more torque to accelerate quickly uphill.
Also, can a car have the same acceleration on a flat surface as an inclined one?
Do more gears=faster acceleration? Please help me clear out my confusion
once and for all!
Thanks!
I've always had a hard time figuring out what I should be looking at in a car to
predict acceleration. It's obvious that more power and less weight will yield
higher acceleration, but should I be looking at torque or horsepower?
I've read that more horsepower=higher top speed and more torque means
reaching that top speed faster, but what difference does it make if the
manufacturers limit the max speed to say 260 KPH?
I've also noticed that two cars with the same 0-60 time can have a hugely
different 0-100 or 0-150 acceleration time.
How about accelerating on a steep gradient? If I understood correctly from
what I've read, then you'll need more torque to accelerate quickly uphill.
Also, can a car have the same acceleration on a flat surface as an inclined one?
Do more gears=faster acceleration? Please help me clear out my confusion
once and for all!
Thanks!
Last edited by drummerdimitri; 10-22-2012 at 04:46 PM.
11-04-2012, 07:57 PM
#2
Possibly the reason for scant reply is that it's a complicated and involved question, being one of those very general questions that touch a very wide array of topics. One could just about write a book about it, and a forum post to answer it can easily become a short novel.
I'll try to keep it simple and curt.
Power to weight ratio is a major consideration for acceleration yes. Even for open chequebook manufacturers like Ferrari and Lamborghini, they make their top line models light.
Tractability is an issue with high power/weight however and so is dynamic chassis balancing. A longer wheelbase can put torque onto the ground better than a short one, for example, since some flex is taken up between the engine and main drive wheels (even on AWD the rear ones do most of the job at launch, drive ratios are typically 30/70 or thereabouts under hard acceleration), leads to more stable drive wheel contact with the road. Things like suspension can also play a role here, you want independent shock absorption at the drive axle but also stabilisation of the torque effect on parts under motivation, the two work against each other a little.
So power/weight and then tractability/handling are two big factors in hard acceleration.
The difference between torque and horsepower is torque is the energy and horsepower is a measurement of energy. One is doing, the other is looking at something doing.
But horsepower involves reciprocal mass as well. Lighter bottom end spins faster (less inertia to overcome), but can't sustain at speed against as much resistance (less inertia to resist deceleration by drag).
By the same token, torque involves induction harmonics and flow dynamics as well and these are typically tuned for low-mid range engine speeds for drivability reasons, otherwise engine idle speeds would be 3000rpm and you'd rev to 4000rpm before moving from the kerb. So the design of passenger engines is tuned to favour torque production over horsepower measurement because the engine isn't spinning fast enough to make a lot of positive inertial mass. But notice how race engines do however, operate under all conditions and at all times at very high engine speeds...
The equation for speed capabilities is drag coefficient versus horsepower. For a typical early 70s compact sedan you need at least 200hp to crack 125mph, any less and the car just won't push through the wind that fast, even if the engine is making enough torque to do it.
As mentioned earlier the design of passenger engines (which are also often the base engines used in motorsport, very few are purposely designed for racing), is geared towards torque production over horsepower production, which is a roundabout way of saying it could produce more horsepower but is detuned for more drivability at low engine speeds. So when you quote torque you're also quoting horsepower potential.
Torque is the actual power that the engine is producing.
Acceleration is all about power/weight and tractability/handling, so it's about torque, the actual power the engine is producing...whilst the horsepower measurement of this potential relates to the impact wind resistance has upon this acceleration. Every car will slow acceleration the faster you go, but the more horsepower you're making at the time the less effect it has. That's why drag racers, which despite being all about acceleration, are all into horsepower production, the mighty powerful engines they use to start with make enough torque for a fast line launch virtually a given, seconds are lost or gained as speeds rise along the quarter mile, most races are won and lost in the latter section of the track past the midway point.
With a gradient your car weight is under different conditions because gravity is straight down through the roof. If you put your car on a slope then gravity will try to push the car down the hill. So there is a little extra inertia to overcome which affects acceleration positively or negatively whether a downward or upward slope. At the same time tractability can be affected since the main drive wheels at the rear get more traction with more weight. Initially you can lose traction on a downward slope or gain traction on an upward one, but you want bigger brakes to handle one and more torque to handle the other.
More gearing can help mechanically tune drivability and acceleration, geared torque raises torque at the drive end (not torque production in the engine), but the issue is engine speeds and induction harmonics, for maximum performance you want gearing which upshifts from the peak horsepower curve to the peak torque curve, so having them too close together can waste time changing gears instead of accelerating under power, whilst having them too far apart can waste time having to bring the engine back up to maximum outputs after upshifting.
About four gears spaced between a tractable first and final drive ratio seems to work pretty well for engine speeds between them, then what you do is increase final drive ratio at the transmission axle (diff) to shift the whole band of gearing down the scale for higher effective torque at the drive wheels. What manufacturers started doing after this became fairly common in production is to put an overdrive 5th gear to make the whole package drive nice on highways too without revving their rings off and burning tons of fuel.
But there were still variations on some of the specific ratios on those four main gears which varied by preference and driving style, so covering all the bases six-speeds came on the scene with 4th slightly under final drive ratio, 5th slightly over final drive (overdrive) for things like highway overtaking, and 6th a long legged overdrive for cruising highways or high speed running. The ratio spacing of those first 4 gears on 6-speeds is very good for almost all high performance applications.
With all that said 3 main gears from 1st to final drive works fine on quarter mile strips, but isn't so good on circuits or road driving. Most of the quarter mile is completed with only one gearchange this way which saves time, but requires a flexible engine setup very smartly. I used a 3-speed with 3.5 final drive on my track car, it worked.
For road or circuit driving again at least four gears between 1st and final drive are really necessary to maintain engine outputs through varied conditions like turns and rises. On my 3-speed I was frequently caught between gears lacking the ratio I needed in mountain driving, turning the car from very fast to very ordinary until I could get the engine speed up again. Case of too fast for 1st, too slow for 2nd, engine spins below the cam/induction harmonics and makes no power.
Even more upon this drivability emphasis are the 6 and 7 speed autos in cars like Mercedes, they're designed to keep the engine at a peak low speed efficiency using variable cam timing, variable intake manifolding and other tuning to create a dual phase, two mode engine. It essentially has a sport mode and a normal driving mode, the normal driving mode economical and comfortable, for low engine speeds. The sport mode maximum power production for high engine speeds. All that gearing can keep the engine ideally positioned in terms of engine speeds in either one or the other.
During hard acceleration however, you'll only use 3 of those gears without losing your license or driving dangerously. In the 7 speed 5th is probably only slightly under final drive ratio and 1st is never engaged unless in sport mode. It adds a lot more features to the old school 4-speed gearing but it's not exactly operating under different rules.
I'll try to keep it simple and curt.
Power to weight ratio is a major consideration for acceleration yes. Even for open chequebook manufacturers like Ferrari and Lamborghini, they make their top line models light.
Tractability is an issue with high power/weight however and so is dynamic chassis balancing. A longer wheelbase can put torque onto the ground better than a short one, for example, since some flex is taken up between the engine and main drive wheels (even on AWD the rear ones do most of the job at launch, drive ratios are typically 30/70 or thereabouts under hard acceleration), leads to more stable drive wheel contact with the road. Things like suspension can also play a role here, you want independent shock absorption at the drive axle but also stabilisation of the torque effect on parts under motivation, the two work against each other a little.
So power/weight and then tractability/handling are two big factors in hard acceleration.
The difference between torque and horsepower is torque is the energy and horsepower is a measurement of energy. One is doing, the other is looking at something doing.
But horsepower involves reciprocal mass as well. Lighter bottom end spins faster (less inertia to overcome), but can't sustain at speed against as much resistance (less inertia to resist deceleration by drag).
By the same token, torque involves induction harmonics and flow dynamics as well and these are typically tuned for low-mid range engine speeds for drivability reasons, otherwise engine idle speeds would be 3000rpm and you'd rev to 4000rpm before moving from the kerb. So the design of passenger engines is tuned to favour torque production over horsepower measurement because the engine isn't spinning fast enough to make a lot of positive inertial mass. But notice how race engines do however, operate under all conditions and at all times at very high engine speeds...
The equation for speed capabilities is drag coefficient versus horsepower. For a typical early 70s compact sedan you need at least 200hp to crack 125mph, any less and the car just won't push through the wind that fast, even if the engine is making enough torque to do it.
As mentioned earlier the design of passenger engines (which are also often the base engines used in motorsport, very few are purposely designed for racing), is geared towards torque production over horsepower production, which is a roundabout way of saying it could produce more horsepower but is detuned for more drivability at low engine speeds. So when you quote torque you're also quoting horsepower potential.
Torque is the actual power that the engine is producing.
Acceleration is all about power/weight and tractability/handling, so it's about torque, the actual power the engine is producing...whilst the horsepower measurement of this potential relates to the impact wind resistance has upon this acceleration. Every car will slow acceleration the faster you go, but the more horsepower you're making at the time the less effect it has. That's why drag racers, which despite being all about acceleration, are all into horsepower production, the mighty powerful engines they use to start with make enough torque for a fast line launch virtually a given, seconds are lost or gained as speeds rise along the quarter mile, most races are won and lost in the latter section of the track past the midway point.
With a gradient your car weight is under different conditions because gravity is straight down through the roof. If you put your car on a slope then gravity will try to push the car down the hill. So there is a little extra inertia to overcome which affects acceleration positively or negatively whether a downward or upward slope. At the same time tractability can be affected since the main drive wheels at the rear get more traction with more weight. Initially you can lose traction on a downward slope or gain traction on an upward one, but you want bigger brakes to handle one and more torque to handle the other.
More gearing can help mechanically tune drivability and acceleration, geared torque raises torque at the drive end (not torque production in the engine), but the issue is engine speeds and induction harmonics, for maximum performance you want gearing which upshifts from the peak horsepower curve to the peak torque curve, so having them too close together can waste time changing gears instead of accelerating under power, whilst having them too far apart can waste time having to bring the engine back up to maximum outputs after upshifting.
About four gears spaced between a tractable first and final drive ratio seems to work pretty well for engine speeds between them, then what you do is increase final drive ratio at the transmission axle (diff) to shift the whole band of gearing down the scale for higher effective torque at the drive wheels. What manufacturers started doing after this became fairly common in production is to put an overdrive 5th gear to make the whole package drive nice on highways too without revving their rings off and burning tons of fuel.
But there were still variations on some of the specific ratios on those four main gears which varied by preference and driving style, so covering all the bases six-speeds came on the scene with 4th slightly under final drive ratio, 5th slightly over final drive (overdrive) for things like highway overtaking, and 6th a long legged overdrive for cruising highways or high speed running. The ratio spacing of those first 4 gears on 6-speeds is very good for almost all high performance applications.
With all that said 3 main gears from 1st to final drive works fine on quarter mile strips, but isn't so good on circuits or road driving. Most of the quarter mile is completed with only one gearchange this way which saves time, but requires a flexible engine setup very smartly. I used a 3-speed with 3.5 final drive on my track car, it worked.
For road or circuit driving again at least four gears between 1st and final drive are really necessary to maintain engine outputs through varied conditions like turns and rises. On my 3-speed I was frequently caught between gears lacking the ratio I needed in mountain driving, turning the car from very fast to very ordinary until I could get the engine speed up again. Case of too fast for 1st, too slow for 2nd, engine spins below the cam/induction harmonics and makes no power.
Even more upon this drivability emphasis are the 6 and 7 speed autos in cars like Mercedes, they're designed to keep the engine at a peak low speed efficiency using variable cam timing, variable intake manifolding and other tuning to create a dual phase, two mode engine. It essentially has a sport mode and a normal driving mode, the normal driving mode economical and comfortable, for low engine speeds. The sport mode maximum power production for high engine speeds. All that gearing can keep the engine ideally positioned in terms of engine speeds in either one or the other.
During hard acceleration however, you'll only use 3 of those gears without losing your license or driving dangerously. In the 7 speed 5th is probably only slightly under final drive ratio and 1st is never engaged unless in sport mode. It adds a lot more features to the old school 4-speed gearing but it's not exactly operating under different rules.
