If I run my car at 3000 RPMs, does it wear out the car any differently if I am in one gear as opposed to another?
How about one speed compared to another?
Is 3000Rpms in 2nd gear at 25mph the same to the engine as 3000Rpms in 5th gear at 90mph?
Freedom wrote:
>Is 3000Rpms in 2nd gear at 25mph the same to the engine as 3000Rpms in 5th gear at 90mph?
That is correct, my friend. Revolutions per minute of the engine crankshaft are measured at the PTO (power take off) end of the crank. 3000 rpm is 3000 rpm no matter what is happening beyond the engine. The only effect the 90mph speed would have would be beneficial… the air-cooling effect on the radiator.
Regards…
vtel57 oversimplifies things. yes, RPMs are what the name says: RPMs. But there are other factors such as load on the motor.
If you are cruising on a flat road and the engine has little load, you are better going up to 5th gear to keep RPMs lower. But once you start climbing, if you stay in 5th gear, the motor will be overloaded and it will go through fewer revolutions but with excessive load so it is better for the engine if you shift down and it will do more RPMs but will have an easier time.
also 3000rpm’s at 70mph is consuming more fuel and producing more power and heat then 3000 rpm’s at 40 mph (or whatever speed you said. so it is diffrent.
K2dave, that statement is highly suspect. Simply put an engine is essentially an air pump, at 3000 RPM it will move a given volume of air, regardless of the gearing or speed. Granted there are exceptions, such as ram-air systems and possibly coasting down a steep hill in gear with the throttle closed. Generally speaking it will create the same power (and heat) at 3000 RPM whether it is in a vehicle traveling 40 MPH or 70 MPH. I am sure someone much more well versed in the theory of the internal combustion engine can better explain this.
As for the OP, 3000 RPM in one gear will wear the engine similarly to 3000 RPM in any other gear given an equal load. As for wear on your CAR, obviously 3000 RPM in 5th gear will create more wear on the running gear than 3000 RPM in 1st or 2nd gear.
Hibbins a ic engine is something like an air compressor. You have to remember that the air intake is restricted and variable. If the engine is at 3k rpms and on level ground at 40mph, it is using little fuel, the air is restricted to match. when you are at 70 or even 90mph (again at 3k) the load due to wind is increased dramatically. you must have your foot further down on the acc. that energy to overcome the wind must come from somewhere. more air is passed through your engine to burn the extra fuel. The amount of air through an engine is not constant at a given rpm.
also if you were to drive at 40mph and are pulling a trailer or driving w/ your brakes on or ac on or windows open you would be pushing more fuel and air thruogh your engine.
You can not tell the amount of fuel or amount of air passing through a ic engine by the rpm’s (possible the air in a 2 stroke but not the fuel).
All gasoline engines, both two and four stroke, adjust both the air and fuel volumes to maintain a proper mixture. The actual ratio varies, but people tend to refer to a an ideal 14.7:1 ratio of air to fuel by weight to keep things simple. I think diesel engines always work with the intake wide open, but newer diesels vary the turbo boost, which is another way of changing the amount of air entering the engine.
It takes more work to move a car at 3000RPM in top gear than it does to move it at 3000RPM in low. The throttle will be open more in high gear, allowing more air into the intake. In low gear, the throttle will be almost completely closed. That’s why low load conditions are also referred to as high vacuum conditions: the throttle is almost closed, and the engine pumps the intake down to a higher vacuum. Since the intake is at a higher relative vacuum, less air by weight enters the cylinders.
A car travelling at 90 mph is using more power than the same car moving at 25 mph - there’s simply no way around that. Power goes roughly as speed cubed, so at 90 you’re using approximately 47 times the power that you are at 25.
This is (nearly) completely independent of what gear the engine is in, or how fast it’s turning, or whatever: to move the car at a given speed requires a certain amount of power - the details of just how that power is generated are left to the internals of the engine/transmission, but the bottom line won’t change.
If 25 mph in 2nd gear corresponds to 3000 RPM, then in order to maintain that speed you open the throttle enough to produce the proper horsepower (you directly meter air, and the PCM decides how much fuel to inject) - producing more than that will cause you to speed up.
Same situation at 90; 3000 rpm, but at a much higher torque (power = torque * angular velocity), which means you’ve got to be dumping more fuel/air into the cylinders, and generating more “thrust” on each power stroke, even though the engine speed is the same as in the previous case.
At 90, you’re burning more fuel per stroke, generating more heat, and placing a higher load on the pistons, connecting rods, crankshaft, etc., than you are at 25 - even though the engine is turning at the same speed in each case.
…BUT…what about the differences between the “highway” and “city” ratings on gasoline-powered vehicles? I was under the impression that travelling at a relatively constant speed (say, 65 MPH) on the highway was far more efficient than stop-and-go traffic travelling at 25 MPH in the city. By your equation, shouldn’t city mileage be superior? If the engine is running at 3000 rotations-per-minute, whatever happens to the power once it moves past the crankshaft and into the gearing shouldn’t matter…right?
My understanding is that City fuel mileage is lower than Highway because in-town driving involves lots of stopping, accelerating back up to 35-40, stopping again a half mile later at the next light, accelerating back up again…
Accelerating from 0-40 at a rate most people find tolerable sucks up a lot of fuel. If you could cruise without stopping at 35 mph for miles on end, I think you’d get extremely good mileage.
I’m kind of at the end of my knowlege rope, so hopefully somebody can step in a clarify or correct me on this. One possibility that comes to mind is that at low speeds (like 25 or 30 mph), aerodynamic drag becomes less important than other power-sucking things (like rolling resistance in tires, or internal friction within the engine), so the fuel mileage is not as low as one might expect going only by power-goes-as-velocity-cubed.
Well, I’m feeling better about being confused by this “easy” question.
The key term everyone is missing is torque. Unlike RPMs, which is just a measurement of rotational speed, torque is a measurement of rotational power. Think of it this way:
An engine spinning at 3000rpms with no load on it is producing very little torque, and therefore consuming a minimal amount of fuel and generating a minimal amount of heat. If you then put a load on the engine and don’t give it more fuel, because the torque is so low the rpms will immediately fall. If the load is great enough they will fall to zero and the engine will stall. To keep it at 3000rpms under a load you must add fuel (i.e. push the throttle). The gasoline will make it generate alot more torque (and alot more heat) even if, because of the load, the engine doesn’t spin at a higher rpm.
Is that even remotely clear?
you also gotta remember that RPMs are only relevant to that engine. 3000 RPM can be much more damaging on a 3-door hatchback then on a Porsche. so really, when 1st gear on a Aston Martin Vanquish is good for about 58mph, that is only relevant to itself. That doesn’t mean you can run a Honda Civic up there, and be fine.
Hail Ants explains it well…
I thought it was power increases with the square of velocity, maybe it is the cube.
either way it goes basically linear until about 45mph then is either the square or cube. this leeds to anywhere from 45-65 mph (depen as the highest mpg speed and is another reason why hwy mialage is greater then city. but the main reason is stop and go driving.
One car I had, had a mpg digital display, the most efficient speed was 63 mph which was about 33mpg if it was accurate which it appeard to be and it also calculated average mpg which was close to the value I calculated when I filled up.
K2dave, I intended to point out that fuel consumption is a factor of load rather than RPM or speed. I did a poor job of it that is clear. My apologies as it appeared I was correcting you, when my intention was to clarify.
I left a key qualifier out of my earlier statement, that being equal load. Guess I am also guilty of taking an overly simplistic view in my response. I assumed WFO throttle and equal load at 3000 RPM, which would give you the same power and heat regardless of speed or gearing.
When I was at the lights once I shifted into neutral (auto gearbox btw) and there was a noticeable increase in the sounds of revs. My mate said to me that I should put it back in gear because it was revving lower but in reality, it’s using the exact same amount of fuel isn’t it?
Even though the revs are lower, the exact same amount of fuel is being used by the idle circuit on the carby isn’t it? The only reason the revs are lower is because the various components of the auto box are putting a load on the engine aren’t they???