Resolute

iTrader: (2)

How about we summarize the relationship between hp, torque, rpm and the loads each place on an engine. This will probably be long. Let's take another look at our super liter engine...
After the last one blew up from revving too high, we decide to build a newer, better super liter engine that makes 100hp. We remember that hp is the amount of work that our engine can perform, and is the result of how much torque it makes and the number of times it makes it. Torque is the amount of force applied to the piston to push it down. This force is created by the explosion of air and fuel (gas), and the more gas there is to ignite, the greater the force applied to the piston. The more often this force is applied, the greater the amount of work the engine performs. In other words, HP increases in relation to torque and rpm. As an example of this, we loaded a car with 100 lbs in a one minute trip. The next time, we loaded the car with 50 lbs in two trips in the same amount of time. In both cases, the amount of work is the same, but the second time we lifted (the force applied) half as much, but twice as fast (rpm). The next trip, we apply a fourth of the force and lift 25 lbs, but move four times as fast. Again, the work performed (HP) is the same. The last trip we lift 50lbs again, and still move four times as fast. The work performed in this case is 200lbs moved vs our original 100lbs moved in the same one minute time frame. So, by increasing both the force and the number of trips per minute, we have doubled our work. The same applies with our engine. If we cut the torque applied on each power stroke of the piston in half, but increase the number of strokes per minute, we can still double the hp performed.
Mathematically it looks like this: HP=(torque*rpm)/5252
Now, on any engine, the amount of gas sucked into the cylinder, and therefore the torque applied due to combustion of that gas, will vary with rpm. The point at which the engine sucks in the most gas is the point at which the maximum amount of torque is applied. On our engine, this occurs at 5000rpm and at this rpm our engine pushes the piston down with 89 lb/ft of torque. Not too shabby. Any rpm less or over 5000, and the torque is less. This is because when the piston travels down the cylinder with the intake valve open at this rpm, it produces a more efficient vacuum than at any other rpm. In our engine, it pulls in one liter of gas, and therefore has a Volumetric Efficiency (Ev) of 100%, at 5000 rpm. At 3000 rpm the piston only pulls in .97 liters of gas with the intake valve open, and so the torque is also slightly less at 87 lb/ft. The rpm at which the Ev is greatest is the rpm at which the engine is able to pull in the maximum amount of air per stroke, and it will make the maximum amount of torque at that rpm.
What determines the rpm at which the Ev is greatest? The piston velocity and duration of the valves being open are the primary factors. The faster the piston moves, the greater the vacuum effect it has to pull air in, and if the valves are open long enough at that rpm, then it will pull in as much gas as possible. This is why the torque increases as rpm rises, because the vacuum created by the piston traveling down the cylinder increases with rpm up to this point of max Ev. Eventually, the piston will move too fast to be as effective and the valves will not be open as long and the amount of air pulled into the engine will drop off. This is why the torque in our engine starts to fall off after 5000rpm to 75lb/ft at 7000 rpm.
So now the torque, or amount of force applied to the piston, is going down, won't HP? Not in our super liter engine it isn't. Remember loading the car? We cut the force in half but went four times as fast and as a result we still doubled the work. The same rules apply to our engine. The load applied to our piston to push it down has decreased to 75 lb/ft, but the rpm has increased and the net result is 100 hp at 7000rpm. Now our little super liter is in the 100hp per liter club and we feel pretty good. But we want more power!
Read on...

Resolute

iTrader: (2)

So we know that if rpm is increased, it can compensate for loss of torque to still make power. Looking for more power, we rev our super liter to 9000rpm and find out that we only have 90hp and 52 lb/ft of torque at this rpm. It turns out that after 7000 rpm, there is not enough time with the valves open for the engine to pull very much gas into the cylinder, and as a result, their isn't very much air/fuel to ignite and push on the piston with. In other words, the torque applied to the piston starts to fall off faster than the increase in rpm can compensate for. We can fix this by getting bigger cams that allow more gas into the cylinder at higher rpm. This will increase the torque applied to the piston at this rpm, and therefore make more hp. but, we decide forced induction is the way to go.
Rather than help the engine pull more air into the cylinder at high rpm, we decide to push more gas into the cylinder. Since we already know more gas will increase the torque, we know pushing more gas into the cylinder will in turn make more hp. The rpm at which the Ev is highest will still be the rpm at which maximum torque is applied, but now that rpm has changed. With forced induction like turbos, the Ev is a combination of the engine's efficiency and the turbo's efficiency. Same for superchargers. our little super liter pulls the most air in at 5000rpm, and as it turns out, our turbo is most efficient at about 3000rpm to 5000rpm with our boost level. Together, our engine now has the maximum amount of gas in the cylinder at 4000 rpm, and so our torque now peaks at this rpm. We also notice that extra air is forced into the cylinder at all rpm over 3000, so torque and hp numbers rise across the rpm range from 3000rpm to our 9000rpm redline. Yea for turbos! If we added a supercharger, the same rules apply. The supercharger will be most efficient at a certain rpm and will usually compliment the engine, so the rpm at which max torque is made might not change, but the amount of air in the cylinders is still raised, and as such hp and torque are still increased across the rpm range of our engine.
So now the super liter is hauling ***** and making 200hp, and we wonder, "is this thing going to blow?" Well, being the experts that we are, we know the tuning is perfect, but how damaging is our extra boost of air? Well, the extra boost of air increases the torque, so we know the loads placed on the piston have gone up. After all, if the load- or "push"- on the piston didn't go up, we wouldn't have any extra torque, would we? Well, the nice thing is that the gas doesn't all explode at once to push our piston down. In fact, as the gas explodes, the piston is already going down, and as the gas continues to burn, the piston continues to go down. This means that the extra force on the piston from our extra air forced into the cylinder doesn't get applied all at once, but over the length of the stroke. So even though we doubled the power, we have not doubled the maximum force on the piston, but applied the extra torque over the length of the stroke. To see this in effect, we measure and see that we have 200hp now at 7000rpm, doubling the power as we said. Of couse, its the load applied as torque that stresses the piston, and we see that at 4000rpm the max torque occurs, and it is 160 lb/ft. So, we have also about doubled the max torque applied, and therefore increased the max compression load on the piston, but it is applied over the length of the stroke and not all at once. This is very nice. So why did our last super liter blow? Because increasing the rpm causes extreme stress on the engine. The faster the piston travels, the faster it must accelerate from a stop to get going again. The higher the rpm, the faster the piston must accelerate to travel the length of the stroke in time, and the inertial loads applied during acceleration increase at an exponential rate to the rpm. Double the rpm, and you increase the inertial loads by 8 times. And unlike the compressive loads from torque that are spread over the length of the stroke, the inertial loads are instantaneous. That means if the acceleration of our piston at 9000rpm from TDC back to the bottom of the stroke, and then again from BDC back to the top of the stroke produce 6000 lbs of force, it is applied all at once at that instant it changes direction. This is not very nice. If the rpm is increased too high, the inertial loads will break the connecting rod. Because of this, we decide revving beyond 9000rpm with our engine is not safe. Taking it one small step further, when the piston travels up past the center of the stroke, it will begin accelerating down. This places tension on the rod. When the piston travels down past the center of the stroke, it will begin to accelerate up. this presses compression on the rod. So our 6000lbs of force are stretching the rod for half the stroke, and compressing it the other half. It is interesting to note, that compression will not fatigue a steel rod, but tension will. Therefore, the tension loads are what will cause eventual failure even if the conrod is strong enough to handle the accelerative loads initially.
And that's engine theory in a nutshell with a look at how it is stressed in the process of making power. Hp is the ability of the engine to do work, torque is the force generated by the explosion of gas in the cylinder, and the rpm is the number of times that torque is applied in a minute. The higher the torque, the higher the stress, but that stress is not applied instantly. The higher the rpm, the higher the stress at an exponential increase, and that stress is applied instantly.
This is all in response to some questions in another thread, but I think a lot of people have the same questions and might find an answer here rather than highjack another thread. Hopefully some people will find all this useful and help them understand the basic relationships between the three variables of an engine's power. Can a moderator make this a sticky?
Will

JoneZZZ

iTrader: (1)

bump...

uwaeve

If this was in response to my post, thanks.

uwaeve

350ztoronto

iTrader: (10)

awsome read thx

Resolute

iTrader: (2)

Since we have already looked at the general relationship of hp and torque and the stresses placed on the engine when we generate them, it's time to add some more cylinders and really see where the magic is.
Now, as discussed earlier, the Mean Effective Pressure (mep) is the average amount of "push" placed on a piston during the power stroke, and this figure is highest when the Volumetric Efficiency (Ev) is highest. When this max value occurs, there is more air/fuel mixture (gas) in the cylinder than at any other engine speed and the torque produced is the highest. HP is the amount of work done and is the byproduct of the engine's torque produced on the crank and the number of times it is produced in a minute (rpm).
Now, this all works out very well and easy with one cylinder, but what happens when we use more than one. As it turns out, more cylinders are better- up to a point. Let's look again at our Super Liter engine. If we connect a little device called a pressure indicator to the cylinder head, we will be able to see the maximum amount of pressure made during every combustion cycle. The combustion cycle is the time from the power stroke to the beginning of the next power stroke, or in the case of a four cycle engine, two revolutions. With our super liter engine, we know that at 5000rpm the Ev is highest, and therefore the mep is highest, and makes the maximum amount of torque at this engine speed. Every combustion cycle at 5000rpm produces 89 lb/ft of torque on the crankshaft. Now, because we can monitor the cylinder pressure, we can see that this 89 lb/ft is not applied evenly across the combustion cycle, but goes up and down as the piston goes through its combustion cycle. In fact, what we have come up with is the average, or mean, torque. The reality is, just after the power stroke begins and the gas is ignited, peak cylinder pressure spikes, and so does the force applied to the piston. If we were to plot cylinder pressure, and thereby torque, as function of crank angle we would see that just after 0 deg, the curve depicting our torque goes sky high and peaks at about 80 deg and then falls pretty fast until it is about 0 at BDC of the power stroke, or 180 deg. It stays pretty flat as the piston comes back up to TDC for the exhaust stroke, or 360 deg and nothing really changes as it goes past 540 deg or BDC on the intake stroke. Then we notice that as the piston completes its combustion cycle and starts to compress the gas as it goes back up to TDC, the torque on the crankshaft actually goes negative! What happens is this: as the piston begins to compress the gas, it loses some of it's energy, and now the force that was once pushing the crank around is now faced with an accelerative force in the opposite direction. This negative accelerative force trying to push the piston back against compression and the loss of torque is the result of engine harmonics. All four-cycle engines suffer from 1/2, 1, and 1 1/2 order harmonics from the unequal application of torque about the crank centerline, and these forces combine to act against the piston. (For those wondering where the half integers come from, it is because there are two revolutions to the combustion cycle, if there were just one, it would be a whole integer harmonic) Nevertheless, the piston makes it up and begins the combustion cycle over again, aided by the law of conservation of angular momentum, which means it has enough energy to make it a couple more cycles before it comes to a stop. That torque is applied in surges, and as a result, is not very smooth, is an inherent problem with four-cycle engines. This is why our super liter vibrates so much while idling. At its highest value, the energy applied to the piston is much higher than the 89lb/ft would suggest, and settles to have a net effect 89lb/ft of torque over the course of one cycle. Now, since we have seen that the torque is applied unevenly across individual cycles, where are the other cylinders I promised? Read on, as the super liter becomes our four cylinder super liter and see how this affects the torque, and therefore stress, placed on the engine.

Resolute

iTrader: (2)

When our super liter is one cylinder, it makes a full revolution without producing any power. In fact, it is losing power to friction and pumping losses as it just rotates. What if we could make another "push" on the crank during this revolution. Well, that is how a two-cycle engine works, but we decide for the sake of the environment that running a gas/oil mix just won't do. We also remember that since power is made by increasing the number of "pushes" used to rotate the crank over a period of a minute, that adding another power stroke during that revolution will help make more power. So we make another cylinder for our super liter. Now, just making another cylinder of the same size would give us a super two liter engine, and that won't due, because we want to race in the super liter series, and to compete displacement is limited to just one liter. So, we now have 2, 1/2 liter cylinders working together. How does this affect our hp? Remember that HP=(torque x rpm)/5252, well we haven't really changed that much. The displacement is the same, and for simplicity, our mep is the same across all the cylinders, and the piston crown area for both cylinders adds up the same as the single piston area in our original engine. So the torque applied over the combustion cycle, since the pressure is the same, and the total piston crown surface area is the same, shouldn't change, but it has. Say what? When we look at a graph of our torque produced over one cycle, we now have two "pushes" on the crank, one for each revolution. This increases the efficiency of the engine, so now the amount of torque applied over the combustion cycle has increased. It doesn't double, but we do get more overall "bang" for the buck, so to speak. On this note, we decide to make our liter engine a four banger and we now have two powerstrokes for every revolution of the engine, or four for every cycle. This means that even though the total piston area hasn't changed, our mep across all the cylinders hasn't changed, and the displacement hasn't changed, our engine will make more power for having four cylinders as compared to just one. We also have a smoother engine, as the torque produced by those evil harmonics discussed earlier are now counteracted by another application of torque on the crank. Now, why not increase the cylinder count to a 20 cylinder one liter? One, because it sounds stupid to say, and two, because like most things in engineering, there's a give and take. By increasing the number of cylinders we increase the friction and pumping losses of the engine. Right now, 10 cylinders is generally about the most efficient before diminishing returns. Of course, there are exceptions. VW's W16 springs to mind. What is important to note, is that a 2.5L six cylinder will make more torque over the course of one cycle, and therefore power, than a 2.5L four cylinder, all things equal. not to mention run smoother as it has three powerstrokes per revolution versus two.
So torque is a force applied over time, or rpm, to produce HP. The more discplacement, the more torque can generally be made, as the more air and fuel there will be to burn. The more cylinders, the greater the amount of torque applied due to increased efficiency, and thereby the greater the amount of hp made. And, an engine making 200lb/ft of torque at a low rpm requires the same energy to develop that torque as the same engine developing 200lb/ft at a higher rpm, what does change is how much hp will be generated. The higher the rpm where torque is made, the higher the hp figures will be. Hope this has been somewhat helpful and not too confusing.
Will

287HP

Great Read! Thanks.

overZealous1

iTrader: (27)

thank god we have intake designs, exhaust designs, camshafts, turbos, superchargers, different bore/stroke relationships, nitrous, flywheels, inline, boxer, timing curves, to manipulate all this info, lol.

btw-the vw w16 in the veyron is sick. 900lbs@2200rpm and a peak power of 1001hp 4 turbos and all wheel drive, if i only had an extra 1.5million!!!