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Why do more powerful cars consume more gas?

fuzzybabybunny

fuzzybabybunny

Moderator<br>Digital & Video Cameras
Moderator
I don't really know anything about cars, so i was wondering:

Two cars, one a 400hp and another a 120hp car. Both are the same weight, same aerodynamics, going the same speed, at the same rpm. Why does the 400hp one get 18mpg while the 120hp one gets 40mpg? Considering the mass/velocity/drag are the same, both should require the same energy output to keep going, no?
 
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV
 
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
 
Originally posted by: Zenmervolt
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV
Click to expand...

Right. If one unit of gasoline is X units of energy, why do two cars that are the same mass and velocity require different amounts of energy? Aren't they both using the same power? Why would the 400hp car be using more energy than the 120hp car? Isn't the 400hp basically untapped unless the car accelerates or has more resistance imposed on it like towing something?
 
Originally posted by: fuzzybabybunny
Originally posted by: Zenmervolt
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV
Click to expand...

Right. If one unit of gasoline is X units of energy, why do two cars that are the same mass and velocity require different amounts of energy? Aren't they both using the same power? Why would the 400hp car be using more energy than the 120hp car? Isn't the 400hp basically untapped unless the car accelerates or has more resistance imposed on it like towing something?
Click to expand...

To make more power a motor needs more air and more air needs more gas. There are only a few ways to get more air. A larger displacement means the pistons themselves have to be larger thus creating more friction and using more gas. A supercharger uses engine power to compress air and create more power, which again creates more friction. A turbocharger is going to be the closest in gas consumption, since all it would do in that situation is create a little extra backpressure.
 
Originally posted by: Eli
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
Click to expand...

This is what I was getting at. I threw in the rpm thing because I figure that if they WEREN'T at the same RPM, ex. if the 400hp engine was at a higher RPM, then it would be using more gas as a result of turning faster. So I decided to keep this equal as well.
 
Yeah. I was just thinking that ANY car can cruise at, say, 30mph. Even a 50hp car could do this. And yet a 400hp one uses so much more gas to do this. And I already knew that all modern engines nowadays are already about as efficient as they're going to get due to the carnot cycle, right? So where does all that extra fuel energy go? It's not being sent to the road, because otherwise it'd be accelerating or going faster or having to propel a larger mass or overcoming a greater drag. But all cars are going the same speed with the same mass and aerodynamics.
 
Originally posted by: fuzzybabybunny
Yeah. I was just thinking that ANY car can cruise at, say, 30mph. Even a 50hp car could do this. And yet a 400hp one uses so much more gas to do this. And I already knew that all modern engines nowadays are already about as efficient as they're going to get due to the carnot cycle, right? So where does all that extra fuel energy go? It's not being sent to the road, because otherwise it'd be accelerating or going faster or having to propel a larger mass or overcoming a greater drag. But all cars are going the same speed with the same mass and aerodynamics.
Click to expand...

Read what I posted, I edited it.

In short, more power comes from three places:

Displacement: Bigger pistons = more friction.
Supercharger: Engine power needed to spin the blower uses gasoline.
Turbocharger: Engine exhaust spins the turbo, this one will be very close to original motors fuel consumption, but a little more gas used due to added back pressure = more force piston needs to make.
 
Originally posted by: fuzzybabybunny
Originally posted by: Eli
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
Click to expand...

This is what I was getting at. I threw in the rpm thing because I figure that if they WEREN'T at the same RPM, ex. if the 400hp engine was at a higher RPM, then it would be using more gas as a result of turning faster. So I decided to keep this equal as well.
Click to expand...

If the two engines were at the same RPM and in the same gear (assuming the gearing is the same), then the 400BHP would be travelling at a higher speed than the 120BHP car. Air resistance, etc. is then introduced. From what I can remember, air resistance (drag) increases as the square of the speed.

Yep, it does: http://en.wikipedia.org/wiki/Drag_(physics)

The power required to overcome the drag is cubed.

If the 400BHP car is in a higher gear then the 120BHP car to keep the same speed then other friction / efficiencies are surely introduced (others more knowledgeable than me can expand on this).

You may find this interesting:
http://en.wikipedia.org/wiki/E...e_.28petrol.29_Engines

Edit: Quote from the above link:
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power a car. In other words, of the total heat energy of gasoline, 70 to 75% is ejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007 Ford Focus, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6,000 RPM).
 
Originally posted by: crazySOB297
Originally posted by: fuzzybabybunny
Originally posted by: Zenmervolt
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV
Click to expand...

Right. If one unit of gasoline is X units of energy, why do two cars that are the same mass and velocity require different amounts of energy? Aren't they both using the same power? Why would the 400hp car be using more energy than the 120hp car? Isn't the 400hp basically untapped unless the car accelerates or has more resistance imposed on it like towing something?
Click to expand...

To make more power a motor needs more air and more air needs more gas. There are only a few ways to get more air. A larger displacement means the pistons themselves have to be larger thus creating more friction and using more gas. A supercharger uses engine power to compress air and create more power, which again creates more friction. A turbocharger is going to be the closest in gas consumption, since all it would do in that situation is create a little extra backpressure.
Click to expand...

Ok, let's leave a supercharger out since it's obvious that this additional equipment would consume more fuel (as would a heavier alternator, A/C compressor, etc). So lets say 1 unit of air + 1 unit of gas creates 1 unit of energy. All cars regardless of engine hp require the same amount of energy to go at the same constant speed.

1 unit of gas stores 1 unit of energy. Does 1 unit of air + 1 unit of gas somehow release less of this stored energy than 5 units of air and 1 unit of gas?
 
Originally posted by: fuzzybabybunny
Originally posted by: Eli
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
Click to expand...

This is what I was getting at. I threw in the rpm thing because I figure that if they WEREN'T at the same RPM, ex. if the 400hp engine was at a higher RPM, then it would be using more gas as a result of turning faster. So I decided to keep this equal as well.
Click to expand...

Not really.

It all depends on gearing, weight and efficiency.
 
Originally posted by: fuzzybabybunny
Originally posted by: crazySOB297
Originally posted by: fuzzybabybunny
Originally posted by: Zenmervolt
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV
Click to expand...

Right. If one unit of gasoline is X units of energy, why do two cars that are the same mass and velocity require different amounts of energy? Aren't they both using the same power? Why would the 400hp car be using more energy than the 120hp car? Isn't the 400hp basically untapped unless the car accelerates or has more resistance imposed on it like towing something?
Click to expand...

To make more power a motor needs more air and more air needs more gas. There are only a few ways to get more air. A larger displacement means the pistons themselves have to be larger thus creating more friction and using more gas. A supercharger uses engine power to compress air and create more power, which again creates more friction. A turbocharger is going to be the closest in gas consumption, since all it would do in that situation is create a little extra backpressure.
Click to expand...

Ok, let's leave a supercharger out since it's obvious that this additional equipment would consume more fuel (as would a heavier alternator, A/C compressor, etc). So lets say 1 unit of air + 1 unit of gas creates 1 unit of energy. All cars regardless of engine hp require the same amount of energy to go at the same constant speed.

1 unit of gas stores 1 unit of energy. Does 1 unit of air + 1 unit of gas somehow release less of this stored energy than 5 units of air and 1 unit of gas?
Click to expand...

No, but the motor had to do work to get that air in, and that energy is wasted. So the engine is then less efficient.
 
1. Higher horsepower engines tend to be larger and have more moving parts that generate more friction.

2. Powerful engines also tend to be heavier and necessitate heavier transmissions, axles, brakes, etc (not always, though). So you have more rotational mass that needs to be accelerated, even if the total car weight was the same in your example.

3. Powerful engines are generally run with throttles that are more closed at any given speed, increasing pumping losses and decreasing efficiency. An engine achieves maximum efficiency at full throttle, and weak engines allow you to use more throttle more of the time.

4. Engine tuning (fuel/air and compression ratios, forced injection, etc).

That said, the difference isn't always as great as your example. The base-model Corvette has a good reputation for pairing stellar performance potential when you mash the throttle with surprisingly good efficiency when you don't mash the throttle. Its highway mileage is probably similar to my S2000 despite weighing more and making ~200 HP more. But that's comparing two cars tuned for power.
 
Originally posted by: daw123
Originally posted by: fuzzybabybunny
Originally posted by: Eli
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
Click to expand...

This is what I was getting at. I threw in the rpm thing because I figure that if they WEREN'T at the same RPM, ex. if the 400hp engine was at a higher RPM, then it would be using more gas as a result of turning faster. So I decided to keep this equal as well.
Click to expand...

If the two engines were at the same RPM and in the same gear (assuming the gearing is the same), then the 400BHP would be travelling at a higher speed than the 120BHP car. Air resistance, etc. is then introduced. From what I can remember, air resistance (drag) increases as the square of the speed.

Yep, it does: http://en.wikipedia.org/wiki/Drag_(physics)

The power required to overcome the drag is cubed.

If the 400BHP car is in a higher gear then the 120BHP car to keep the same speed then other friction / efficiencies are surely introduced (others more knowledgeable than me can expand on this).

You may find this interesting:
http://en.wikipedia.org/wiki/E...e_.28petrol.29_Engines

Edit: Quote from the above link:
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power a car. In other words, of the total heat energy of gasoline, 70 to 75% is ejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007 Ford Focus, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6,000 RPM).
Click to expand...

More powerful cars consume more gas because people tend to accelerate quicker because they can. Given your scenario of identically efficient engines, one having the capacity to make 400hp and the other much less, they would indeed use an identical amount of fuel. The response I quoted is incorrect, the first line regarding speed is incorrect. Horsepower has zero correlation with speed as it relates to rpm, I dont know how you come to the conclusion that in an identical gear (which would assume identical gear ratio) and at identical rpm the cars are capable of travelling different speeds, this is wrong.

Then next line you talk about consuming more fuel at any given rpm point simply because it makes more power. This as well is incorrect, the possibility of achieving 400hp does not mean that you are currently utilizing 400hp, or that fuel is being supplied to support 400hp. Given identical air flow (which would relate to an identical acceleration), the cars would consume the same amount of fuel.

There is no extra energy to be wasted, if there were, it would result in additional acceleration.

Again, the concept that the 400hp car would be moving faster all other variables being equal is incorrect.
 
I'm not positive but quite sure it has nothing to do with friction and pretty much everything to do with efficiency. It's well known that the best use of any given amount of gasoline in generating its power in an engine will be when the engine is at full tilt; i.e. a 50 horsepower engine is simply more efficient at doing 50 horses of work than a 100 horsepower one.

A CC of fuel air combination at a higher pressure will generate more work than at a lower one. Forget the equal RPM for a sec but if you have a 120 horsepower, small engine producing the power required to keep you going at 70 mph it's going to be stressing itself more than the 400 hp one keeping the car going at 70. Its fuel air mixture has to be more aggressive. Obviously both engines are doing exactly the same amount of work, friction losses notwithstanding, but the 120 is simply better at this work because it's closer to its maximum work capacity and the closer an engine gets to that the more energy it can pull out of a given unit of gasoline.

thomsbrain corvette example does so well on the highway because its longest gear lets the RPM go extremely low, which means that to produce the same amount of work required to maintain the 70 mph each given revolution has to have more power in it and thus the engine is trying harder to each RPM. Take any vehicle in 1st gear at 25 mph and now the same car at the same 25 mph in 5th gear. The 5th gear does better on fuel because its rpm are much lower and each revolution thus has to do more work, bringing it closer to the engine's preference to reach most efficiency when it's working hard.
 
Originally posted by: Skoorb
I'm not positive but quite sure it has nothing to do with friction and pretty much everything to do with efficiency. It's well known that the best use of any given amount of gasoline in generating its power in an engine will be when the engine is at full tilt; i.e. a 50 horsepower engine is simply more efficient at doing 50 horses of work than a 100 horsepower one.

A CC of fuel air combination at a higher pressure will generate more work than at a lower one. Forget the equal RPM for a sec but if you have a 120 horsepower, small engine producing the power required to keep you going at 70 mph it's going to be stressing itself more than the 400 hp one keeping the car going at 70. Its fuel air mixture has to be more aggressive. Obviously both engines are doing exactly the same amount of work, friction losses notwithstanding, but the 120 is simply better at this work because it's closer to its maximum work capacity and the closer an engine gets to that the more energy it can pull out of a given unit of gasoline.

thomsbrain corvette example does so well on the highway because its longest gear lets the RPM go extremely low, which means that to produce the same amount of work required to maintain the 70 mph each given revolution has to have more power in it and thus the engine is trying harder to each RPM. Take any vehicle in 1st gear at 25 mph and now the same car at the same 25 mph in 5th gear. The 5th gear does better on fuel because its rpm are much lower and each revolution thus has to do more work, bringing it closer to the engine's preference to reach most efficiency when it's working hard.
Click to expand...

Not to nit pick, but additional friction would be adding to the inefficiency of a larger motor.

The easiest way to explain it would be using a 4 vs. 8 cylinder motor and Think of each cylinder as a person. 8 people can push a car easier than 4 and 8 will eat more food after pushing said car.
 
Maybe I should have said "little" to do with it? 🙂 The closest I've ever come to tearing apart an engine is taking off an intake manifold! However, if friction was really appreciable enough to literally steal multiple MPG for the work done I believe that heat and wear would transfer quickly into failing parts and bad tolerances. A well lubricated engine surely should feel extremely free in its motion.
 
Originally posted by: Skoorb
Maybe I should have said "little" to do with it? 🙂 The closest I've ever come to tearing apart an engine is taking off an intake manifold! However, if friction was really appreciable enough to literally steal multiple MPG for the work done I believe that heat and wear would transfer quickly into failing parts and bad tolerances. A well lubricated engine surely should feel extremely free in its motion.
Click to expand...

an 8 cyl motor has 8 sets of rings scraping a cyl wall for compression a 4 only has 4. How can that not be a much larger amount of friction?
 
Originally posted by: Skoorb
Maybe I should have said "little" to do with it? 🙂 The closest I've ever come to tearing apart an engine is taking off an intake manifold! However, if friction was really appreciable enough to literally steal multiple MPG for the work done I believe that heat and wear would transfer quickly into failing parts and bad tolerances. A well lubricated engine surely should feel extremely free in its motion.
Click to expand...

You are correct, friction has very little to do with an engine's efficiency. Sure, an 8 cylinder engine has more drag than a 4 cylinder, that is a given.. but it is more or less negligible for purposes of this discussion. I'm not even sure if "friction" is the right term, it's more due to oil viscosity drag.
 
Originally posted by: RGUN
Originally posted by: daw123
Originally posted by: fuzzybabybunny
Originally posted by: Eli
My knee jerk reaction was the same as all the other posters, but after reading his question, it's actually a good one.

The thing that kills his question is the "at the same RPM" part. If he had left it at same speed, it would be an even better question. A 400HP engine should be able to turn a lower RPM at speed than a 120HP engine, helping to round out the MPG numbers.

Anyway, since all engines are more or less around the same efficiency, the answer is displacement. A 400HP engine is going to displace much more air than a 120HP one, therefor it is going to require more fuel at any given RPM. You can't just scale fuel use back while still pumping the same amount of air, this will result in a lean running condition.

I dunno. I'm not exactly sure what the answer to the question is. I guess the extra energy is just wasted.
Click to expand...

This is what I was getting at. I threw in the rpm thing because I figure that if they WEREN'T at the same RPM, ex. if the 400hp engine was at a higher RPM, then it would be using more gas as a result of turning faster. So I decided to keep this equal as well.
Click to expand...

If the two engines were at the same RPM and in the same gear (assuming the gearing is the same), then the 400BHP would be travelling at a higher speed than the 120BHP car. Air resistance, etc. is then introduced. From what I can remember, air resistance (drag) increases as the square of the speed.

Yep, it does: http://en.wikipedia.org/wiki/Drag_(physics)

The power required to overcome the drag is cubed.

If the 400BHP car is in a higher gear then the 120BHP car to keep the same speed then other friction / efficiencies are surely introduced (others more knowledgeable than me can expand on this).

You may find this interesting:
http://en.wikipedia.org/wiki/E...e_.28petrol.29_Engines

Edit: Quote from the above link:
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power a car. In other words, of the total heat energy of gasoline, 70 to 75% is ejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007 Ford Focus, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6,000 RPM).
Click to expand...

More powerful cars consume more gas because people tend to accelerate quicker because they can. Given your scenario of identically efficient engines, one having the capacity to make 400hp and the other much less, they would indeed use an identical amount of fuel. The response I quoted is incorrect, the first line regarding speed is incorrect. Horsepower has zero correlation with speed as it relates to rpm, I dont know how you come to the conclusion that in an identical gear (which would assume identical gear ratio) and at identical rpm the cars are capable of travelling different speeds, this is wrong.

Then next line you talk about consuming more fuel at any given rpm point simply because it makes more power. This as well is incorrect, the possibility of achieving 400hp does not mean that you are currently utilizing 400hp, or that fuel is being supplied to support 400hp. Given identical air flow (which would relate to an identical acceleration), the cars would consume the same amount of fuel.

There is no extra energy to be wasted, if there were, it would result in additional acceleration.

Again, the concept that the 400hp car would be moving faster all other variables being equal is incorrect.
Click to expand...

I agree with some of the stuff you posted.

My previous post was made when I was drunk and I agree that a lot of it was bullsh*t.
 
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