Car companies advertise the power of an engine at different rpms.
Car 1: 80 PS at 6000 rpm
Car 2: 85 PS at 6500 rpm
What does the power at different rpm mean? Does it mean Car 1 has better power than Car 2 because it's at lower rpm?
Or does the rpm not affect the power comparison?
What does the power at different rpm mean? Does it mean Car 1 has better power than Car 2 because it is at lower rpm?
It depends.
Or does the rpm not affect the power comparison?
tl;dr: The rpm of the power peak affects the engine's usability for different applications.
The "peak power" number is just one point on the power band of the engine. Ideally, you'd like to know the entire curve (and how its affected by ambient air temperature, altitude, humidity, phases of the moon, etc):
In this figure, we see the horsepower and torque curves for two engines (where torque is solid and horsepower is dotted). Remember, torque is the engine working on the axles and, thereby, the wheels and tires. This is what accelerates the car forward.
If you know the torque of the engine at a particular rpm, you can calculate the horsepower at that rpm:
horsepower = (torque * rpm) / 5252
Admittedly, the above equation is specific to crazy old-time Imperial units. Insert appropriate conversion factors if you'd like to get metric units out instead.
Very casually, you can imagine horsepower (or "power" if you're being unit agnostic) as being "that which maintains speed in spite of drag." We generally expect a high horsepower vehicle to have a higher top speed (provided that it has the gearing to reach that speed).
Looking at the above equation, you can also see that, in these units, the horsepower and torque curves always cross at 5252 rpms (i.e., the scalar values are equal even though they're totally different units).
So what?
All of the above helps you understand what the manufacturer is telling you a bit better. What's missing is "what do you want from a car?"
Going back to the chart, you can see that the blue line has a torque peak at about 2500 rpm and that it doesn't drop off until about 4000 rpm. This means that, from a stop, the car will feel like it pulls away strongly, right away. However, as the rpms get much higher, the engine will seem to run out of breath, accelerating much slower at 6000 rpm than it was at 1000 rpm. Qualitatively, this is what we would expect from a large-ish displacement normally aspirated engine.
The red line has a torque peak at about 5500. It will feel sluggish from a stop and seem to wake up as the rpms increased. From 5500 to 7500, the red engine will out-accelerate the solid engine by a significant margin. This is roughly what we would expect from a smaller engine (and forced injection would only increase this late-rpm peak).
The question for the customer is: which do you like better?
Qualitative summary:
The early torque peak of the solid line is fun from a standing start but you will need to shift early (trading mechanical advantage to get back to the torque peak). Hopefully, you have enough gears to get you to top speed. This profile is often preferred in a street car.
The later peaks of the dotted line is sluggish from a standing start but becomes progressively more exciting as revs increase. You won't need to shift as early to stay at peak power, keeping the mechanical advantage for more revs. This profile is often preferred in a race car.
Full disclosure: I've owned low-end, high-end and (high-end + turbo) vehicles and I prefer the last combination. I haven't been interested in standing starts from a red light for many years.
I pick the "flexible" engine, in the above chart, because in the 1000 to 2500 rpm range it has up to 30% more hp and in the 2500 to 4500 rpm range it has up to 15% more HP. Why do I prefer this? Because 99% of the time I will be within these RPM ranges, the engine will be producing more horsepower without a lot of noise or fuss, with better engine longevity - the engine will be producing more power at a lower RPM hence less engine wear, and finally there is more instant acceleration when I need to accelerate without having to change down a gear every time, which I would have to do in the peaky engine. Though not often stated, an engine with double the torque at a specific RPM has double the HP at that same RPM. The question is do you prefer to have the HP when the engine is at screaming high RPM or do you prefer to have it when the engine is running at more moderate RPMs. Trouble is a lot of people look at the peak HP figure and think that will determine how strong the engine feels/accelerates at the RPMs most people drive at (that is the 1500 to 4000 RPM range), and will often be disappointed. Those who argue the peaky engine (with higher peak HP) is better, will often be those who don't mind reduced engine life, higher noise. My 10 year old son, thinks cars with broken mufflers are the "sportiest" and coolest. I think one's engine choice above will often be determined by which category one falls into, those categories being the "coolness" versus the "sensibility" category.
You don't need to worry about the RPM, just what the engine is capable of putting out. Engine 2 has more power, but together with differences in gearing and everything else, the fact that it does more RPM doesn't mean much. It could just be that the engine puts out about the same force as car 1 (Torque) but turns slightly faster therefore it has slightly more power. Power isn't actually how "strong" your engine is, that's torque. Power is essentially torque multiplied by how fast the engine turns. But aside from technicals, the RPM at which is produced its max horsepower doesn't matter much. There are more important things like the shape of the torque curve, etc. that they don't tell you.
The RPM does not affect the power comparison.
Based purely on what you said, Car 2 would have the engine better no matter what the RPM ratings were. It could be 85 PS at 3000 rpm and it would still be better (and probably a lot more torque-y!)
That's what gearing is for.
If car 2 has a ratio 6500/6000 times = 13/12 times the gear ratio of car 1, it will provide equivalent power and force at the wheels.
Some people say you should be looking for engine torque. Actually, you should be looking for power, because torque is multiplied by the gear ratio, but power stays constant. So, if you know the power and the wheel speed, you will know the torque at the wheels. You do not know the torque at the engine, unless you know the gear ratio.
The people who say you should be looking for engine torque have something right, however: if shifting is inconvenient (due to e.g. driving a manual transmission) and you care about fuel economy, it is useful that the maximum power RPM is as close as possible to the cruising RPM. Also there will be slight shifting delay, although modern automatic transmissions and CVTs make the delay short. Note that looking for torque will logically lead to turbocharged engines and especially turobdiesels, which have their own peculiar problems. I would without any hesitation pick a modern high-revving VVT non-direct-injected naturally aspirated gasoline engine instead of a direct-injected torquey turbodiesel, if I have to pay for the car maintenance at the end of its life.
My suggestion? Get the more powerful car if both have an automatic transmission. If not, you should be seriously considering automatic transmissions, because the automatic shifting makes all of the engine power very conveniently available just by flooring the accelerator. With a very slight delay, of course.
Of course, all of this assumes the cars have equivalent weight. If car 1 is lighter, it may actually accelerate quicker.
So, instead of looking at power, you could be looking at acceleration. The standard is 0-100 km/h time, but I believe something like 50-120 km/h would be more relevant in real life use cases.
The deciding factor of an engines output is BMEP. Brake mean effective pressure. The dimensions of the engine components such as crankshaft throw, cylinder bore diameter and stroke will decide the BMEP. Further consideration would be volumetric effciency, ignition timing and the number of cylinders. An engine may be designed for economy, or power, or simply to have dimensions to fit under a bonnet or a transmission assembly. So the output will vary for what the engine was designed to do, small economy vehicle or muscle car. The most widely accepted measure of an engines output is the kilo Watt. A greater number of kilo Watts does not always mean a better engine, it is design dependant. One horsepower is equal to 0.746 kilo Watts. 1 PS is slightly less then a 1 horsepower. PS is a German measurement which together with the horse power that has fallen out of general use but keeps on appearing. Motor manufactures will always use descriptions of their product which will be the most seductive to their target audience to maximise sales.
Engine power specification changes with the RPM specification as per the formula HP = TORQUE x RPM รท 5252 As the size of the engine increases, more power is created through lesser revolutions of the motor shaft
