Obviously all the cylinders in an engine are used to move the vehicle. However, While driving to work today and needing to put the pedal to the metal to merge I got to wondering just how much power was really available to me from my six cylinder engine. I’m using the example of a four-banger here to simplify things.
Basically, if I’m understanding car engines correctly (which admittedly I’m no expert on), a car engine runs on a four-stroke cycle.
Cylinder moves down and draws in air and fuel.
Cylinder moves up, compresses fuel/air mixture.
Cylinder gets pushed down when the fuel/air mixture ignites thus providing power to the crankshaft.
Cylinder moves up pushing the burnt exhaust gasses out of the engine.
If I’m correct in that list the only time power is provided to move the car is in step #3. I would assume that a four-banger has the cylinders staggered so that one piston is always on each step (so there is constant power is applied to the crankshaft).
However, in essence this means that your car is only being pushed by one cylinder at any given time. Is it reasonable to assume then that if my four-banger is a 240HP engine that each and every cylinder generates 240HP (go with the example even if a 240HP 4-cylinder is silly since it helps for the next bit)?
Now let’s take an 8-cylinder engine that also produces 240HP. I would assume that I now have two cylinders in each stage listed above. Does that mean that each cylinder produces 120HP? Usually, fuel efficiency aside, more cylinders are considered better but in this example is there anything better to the 8-cylinder engine than the 4-cylinder if they have equal horsepower? (Assume cost isn’t an issue and all other things are equal…at least as much as possible for comparison’s sake).
Re-reading my post it looks like I didn’t have to use a 240HP engine as an example afterall but since it is divisible to whole numbers by 4, 6 and 8 just stick with it as it might still be useful in later answers.
Dang…I guess to be accurate I should also replace ‘cylinder’ with ‘piston’ throughout most of my post. I do understand the difference so just write this one down to a brain fart.
HP is rate of energy usage or production. Thus each piston is on average producing 60 HP. For the millisecond it is pushing, it is probably producing more than 240 HP, as work needs to be done compressing the other cylinders etc
You’ve got it right Whack-a-Mole - each cylinder generates power during only a fraction of its cycle. In fact, the peak instantaneous power generated by each piston is probably a good deal higher than the average power output from the entire engine, since it doesn’t generate a constant power output during the entire expansion stroke.
It’s important to keep in mind the difference between average power output and peak power output. In a four cylinder engine each cylinder produces an average of one quarter the engine’s total power output - but because that energy is generated during less than one quarter of the time, each cylinder has a peak instantaneous output greater than the average output from the entire engine. And during the rest of the time, that cylinder is generating negative horsepower, as it takes work from the rest of the engine to pump out the exaust and pump in and then compress the fuel/air mixture.
The answer to you question is yes, sorta, with some caveats.
This might make more sense if we recast your question slightly and talk about torque output of the engine (or cylinder) rather than HP. This isn’t a big stretch, since HP = speed(rpm) X torque (ft-lbs) / 5252; thus, at a constant engine speed, HP is directly proportional to torque.
Torque output from an engine comes from the pistons and connecting rods bearing down on the crankshaft, where torque = force (combustion pressure X piston area) times distance (horizontal distance from crankshaft centerline to conrod end). Since the pressure in the cylinder is constantly changing, and the crankshaft angle is always changing, the torque output from any cylinder is constantly changing.
In fact, if you put a torque cell on an engine crankshaft to measure its torque output, you’ll see the instantaneous torque output vary wildly. It’ll even go negative at some times. But, since all the objects in the drivetrain have some springiness, and some inertial mass, all these torque pulses get smoothed out (pretty much) by the time the power gets to the tires.
So, while the torque output of an engine might be 300 ft-lbs or so, this represents an average number over the entire cycle. And, at any instant, an individual cylinder will be contributing anywhere from, say, -50 ft-lbs (compressing the air takes work!) to +600 ft-lbs of torque [note: I kinda made these numbers up]. But over the entire cycle, you’d expect each cylinder in a four-cylinder engine to contribute an average of 75 of the total 300 ft-lbs.
So if I’m hearing correctly when I mash the accelerator on a 240HP 4-cylinder engine I am really getting only 60HP (on average) pushing me forward? That sucks…more marketing hoopla it would seem.
Let me ask this another way. Assume I have a trailer that requires 200HP to accelerate it at 10 feet per second (I’m just making these numbers up but I don’t think they matter except for illustration). I hook my 240HP, 4-cylinder car to this trailer and step on it. Will I get my 10 feet/sec/sec or something less or something more? (Assume perfect traction)
Uh, no. The total average horsepower output of the entire engine is still 240HP. Each cylinder is producing 60HP, when you average over the entire 4-stroke cycle. The peak power generation inside each cylinder is greater than 240HP for less than a quarter of the time, but you don’t care about that. What you care about is the total average power out of the entire motor, because that’s what determines how fast you can go and how much cargo you can pull.
Sorry AndrewL but you’ve confused me again (admittedly not too hard to do). You said each cylinder produces 60HP yet say the entire engine output is 240HP.
Keeping it simple (ignoring varying power curves and so on and sticking with averages) at any given moment only one piston is providing power to the crankshaft on a 4-cylinder engine. Once that piston is done pushing another takes over. So, each piston on my 4-cylinder engine only provides 60HP so there can never be more than 60HP applied to the cranshaft at any time with this engine (again ignoring peak power outputs and staying with the average).
What am I missing?
[sub]NOTE: I appreciate your precision zut but I need to stick with horsepower right now since pulling torque into this right now is beyond me. I’m just trying to get the sense of this so hopefully ignoring torque for the moment won’t skew the answers out of proportion. However, if it does feel free to chime in again.[/sub]
One other question which I brought-up earlier but wasn’t really addressed is what is better between an 8-cylinder and 4-cylinder engine of equla power outputs?
If I’m following all of this correctly (which I’m not certain I am) each piston in an 8-cylinder engine of 240 total HP would generate 30HP. My 4-cylinder is getting 60HP per cylinder. Earlier I assumed that the pistons in an 8 would go up and down in pairs but now I think on it I suppose they would be staggered so there are two piston in any given step of the engine cycle and that they would likely be half-a-step apart (all guesses on my part so correct me if I’m wrong).
What implications does that have on performance? Ignoring costs and such would I rather have an 8-cylinder or 4-cylinder 240?
Think of the horsepower this way. Imagine you’re at a race track with four very fast cars. You get in the first car and tear off at 240 miles per hour. But it has a very small gas tank, so it can only keep up that speed for one minute. So after that minute, you instantly switch to the second car, going 240 mph. And you keep this up for an hour, using each car in turn.
You’ve been travelling at 240 miles per hour. But each car has only travelled 60 miles (for an average of 60 mph).
Cylinders in an engine are the same way. Each one produces a lot of power, but only for a moment. The time it spends getting ready for the next burst lowers its average. In your example, one cylinder can produce 240 horsepower during its power stroke, but over the course of time it’s only averaging 60.
Not quite. In an 8-cylinder, you don’t have pairs of cylinders firing in sync, they’re offset. One cylinder fires, then when it’s halfway through it’s downward stroke the next cylinder fires.
And as far as I know, every piston engine is designed to fire one cylinder at a time, evenly distributed around the rotation of the crankshaft. V-12, straight-8, flat-6, doesn’t matter. All the old radial engines I’ve seen (the kind on WWI fighter planes, with the pistons spread around the crankcase like spokes on a wheel) have an odd number of cylinders[sup]1[/sup] so they can stagger the firings correctly.
If anyone knows of an engine that wasn’t designed that way, I’d love to hear about it.
[sub]1. Single row radials only, but when they add banks of cylinders they’re offset so the cylinders still fire evenly throughout the rotation.[/sub]
Thanks RobotArm…I think I got it but I’ll rephrase here so you (or anyone) can check to see if it is so.
Each piston in a 4-cylinder, 240HP engine applies 240HP to the crankshaft when it’s in its power stroke. However, the piston has three more strokes to go through before its next power stroke hence it in essence has provided 60HP at each stroke it makes but that is just an average across all strokes. The crankshaft is getting a constant 240HP at all times from the engine (again on average).
Yes. Basically each piston can be thought of as a 240HP motor that’s only on one quarter of the time. The power output averaged over time of each cylinder is 60HP - 240HP 25% of the time, 0HP 75% of the time. The important distinction is between peak power (240HP) and average power (60HP)
Yep, pretty close. Three potential sources of confusion, though:
Make sure not to confuse the four strokes with the four cylinders. For instance, each cylinder of a 240HP, six-cylinder engine produces 160HP over it’s power stroke.
Be sure to remember that the HP measurement (and the torque, for that matter) is time-averaged, so that the 240HP or 160HP or whatever that each cylinder is putting out is an average over that stroke.
And the one truly confusing thing to remember is that it takes power to run the cylinders that are not in their power stroke, because you still have to accelerate the piston and compress the air and so forth. So, really, the single piston in your four-cylinder, 240HP engine is probably putting out something like 255HP, with 15HP pumping the other cylinders and the remaining 240 driving the car. But you still get an average for each cylinder of 60HP over all four cycles.
As to whether you’d be better off with a 4 or 8-cylinder engine of equal power, well, you never really have that choice. There just seem to be too many other factors that kick in. Nobody makes really big 4-cylinder engines because they have a vibration inherent in the design. (I’ve never understood exactly why.) And the pistons would be larger and heavier which would make it less responsive and require stronger connecting rods, and on and on. I’ve never seen a really small V-8, either. You’d have twice as many parts but I don’t think they’d each be half the weight of a 4-cylinder, so it’s lighter, simpler and cheaper to make a 4.
I read an article several years ago and I wish I still had it. It was about technology in Formula 1 racing. Several years ago F1 came up with their current rules that limited engine size to 3 litres. Before that they were all 1.5 litre turbocharged, so all the engine designers had to start from scratch. And they came up with different designs. There were teams trying V-8, V-10 and V-12 engines, and over the years it just turned out that a V-10 worked the best. It had to do with the power output, respnsiveness, light weight (and keeping it low to the ground) and a hundred other things. Those guys would move mountains for a tenth of a second per lap.
Passenger cars have other things to worry about. Think of an inline-6 engine. It’s perfectly balanced, but it’s so long that it’s hard to fit in a modern car. Over the last twenty years the trend has been to make the engine compartment as small as possible so there’s more room inside the car. A V-6 can be packed in a little more tightly under the hood and the only folks who still make a straight-6 are BMW and Jeep.
I remember reading about a motorcycle racing engine that was nicknamed the ‘big bang engine’, all the cylinders would fire in one revolution, then it would ‘coast’ through the next revolution. But this was a one off design, strictly for racing. I couldn’t find much info on it but did get this mention of it in a quick search http://www.motorcycle.com/mo/mcdaily97/day0449.html
Horsepower measures work over time, it is almost meaningless to try to measure how much power each cylinder generates. If my math is right at 3000 RPM each cylinder fires 25 times a second.
Ah, yes, I should have caught that. I was thinking of an inline-4 specifically. I’ve read that it has unavoidable secondary vibrations, but I’ve never found out exactly what causes them. (Although you can add another rotating shaft to the engine just to cancel out the vibration.) It must be something that isn’t there in a horizontally-opposed engine.
Interesting about that big-bang engine, Ana. I’ve heard of some other oddities on motorcycle engines, too, like oval cylinders and pistons. Maybe you can get away with some weird things if you keep them small enough.