Why Can't Pushrod Engines Rev High?

Hello everyone and welcome in this video we're gonna be talking about why pushrod engines don't tend to rev very high and to discuss this we're gonna be looking at a disassembled single cylinder pushrod engine as well as a 3D printed. Chevy LS3 V8 pushrod engine okay so before we get into why these engines don't tend to rev very high we need to understand how they operate so here we have our piston which of course is going to be moving up and down forcing this crankshaft which it's attached to to rotate so the crankshaft is ultimately what's going to be sending power to your transmission so this piston is moving up and down and as it moves up and down it forces this crankshaft to rotate now geared to the crankshaft is the camshaft so it's geared at a two-to-one ratio so for every two rotations of the crankshaft this camshaft will rotate once so you'll see that's geared there and as that crankshaft rotates it forces this camshaft to rotate which you can see has these lobes on it which are what are going to be opening and closing the valves, so you can see the lobes on the camshaft and then we have a cam follower you could also call it a valve lifter or a tappet which will be following that profile of the camshaft so as that camshaft is rotating it will force this lifter up you can see as it follows that profile and then as it continues to rotate that lifter will come back down with it, so it pushes it up when it reaches the peak of that lobe and then it comes back down. Now the tappet is what is going to be moving the push rod up and down so you can see that these will join like so and as that camshaft rotates the tappet will force the push rod to move up and down the push rod then pushes up against the rocker arm which forces the valve to open so you can see that this rocker arm will get pushed up, and as it pushes up it's attached to the valve on the other end and so as it pushes up it forces that valve to move down so looking at the whole interaction you have your camshaft rotating that presses on the tappet which forces the push rod up the push rod forces the rocker arm to rotate and then as that rocker arm rotates it forces your valve down allowing for the airflow to either go in or out so now looking at the cylinder head here we have the push, and you can see that as you push. This push rod down it forces that valve to open so you can see the valve opening right there as that push rod is moved and that's because it's rotating this rocker arm here on the top of the cylinder head, which is pushing the intake valve open and closed, okay, so now that we understand how this system works. We can get into the first reason why push rod engines don't tend to rev very high and that is valve float and so what happens is once you start getting into those higher rpms that means this push rod this entire assembly is having to move back and forth very quickly so these are reciprocating mass rather than rotational mass so it starts to go back and forth very quickly and you have all of this mass within the system that's trying to change directions very quickly as you get into high rpm now the only thing that's keeping this valve from following the cam profile is this spring right here so this spring forces the valve to come back up once you go back to the flat portion of the cam shaft when you're not on that lobe that extends out and so this spring is what's doing all that work to make sure that this entire system remains in contact with this cam lobe but as you get into the higher rpm this reciprocating mass the spring doesn't tend to keep up with it and so as a result you'll have moments where no longer is your tappet following that cam profile perfectly because this spring isn't keeping up with what you're demanding and so this push rod this tappet all of this the valve itself has weight and it's of course reciprocating mass and so all of that mass is trying to go this way, and then the spring is trying to push it back, and if it doesn't maintain contact and it's not able to push it back quickly enough that means as this rotates around you lose contact with the cam profile and there's a moment there where your valve opening is not related to your cam profile, and that's called valve float because you're no longer controlling your valve lift and duration using the camshaft now it's just relying on this spring to come back. So what happens if your valve is no longer following the cam profile? Well, you know best case you just kind of have an efficiency loss things just don't work out in your favor but worst case you could have this valve then contacting the piston, which is on its way up and so you could have some severe engine damage if that were to occur but overall you're not controlling the airflow the way the engine was designed to and as a result you're going to lose efficiency and at higher rpm it's not as easy to get that air flow and so ultimately valve float because of all of this reciprocating mass which you don't have in other style engines such as overhead camshafts you're going to have run into that barrier where valve float is a very real challenge to deal with now you of course can use stiffer springs that can help you with maintaining contact with that cam profile up that of course is an efficiency loss because it takes more work then to compress that spring you're going to have additional losses using a stiffer spring so you may say okay well NASCAR has engines that are using push rods and they're revving to 10,000 rpm, which is indeed true and of course using very stiff springs but of course in Formula One you're seeing you know rpms as high as 20,000 rpm and that's a result they're actually using air spring so rather than a steel coil spring they're using pneumatic springs using air pressure to force that valve up which is easier to control rather than using a steel spring which starts to vibrate, and then you start losing contact with this cam profile now that's of course also due to regulation so you should always take you know rpm range limits in racing with a grain of salt because you know they're restricted within the rules that they can be in so NASCAR could perhaps Rev higher, Formula One could perhaps Rev higher but there are limits set in place that limit them to those peak rpms, but generally speaking dual overhead cam solutions using pneumatic valves are going to be able to Rev quite a bit higher than using a push rod setup which has all of this reciprocating mass. Okay the next reason why push rod engines don't tend to rev very high comes down to air flow because at high rpm you need lots of air and while it is possible for a pushrod engine to use more than just two valves per cylinder it's quite a complex design and so most often you will see two valves per cylinder in pushrod engines and so what this limits it is its airflow at higher rpm so at low RPM there's actually nothing wrong you can benefit from having less valves because it's going to increase the velocity of that intake charge coming in you'll have better fuel mixing but as you get into higher rpm you of course need more air and engine revving at 10,000 rpm versus 5,000 rpm ideally is going to be pulling in about twice as much air because it's revving twice as fast and has twice as many combustion cycles occurring and so by increasing the number of valves you increase the area at which air can flow into the engine and thus how much air you can pack into it in that short duration when you're revving it super high rpm now pushrod engines will often try to mitigate this with the cylinder head design and use valves that come down quite a bit more to allow for more airflow but ultimately a four valve system is going to be able to flow more air through it at higher rpm versus a two valve system, okay, so looking at this 3d printed Chevy ls3 v8 pushrod engine we can get into our final reason why they don't tend to read very high and that comes down to valve timing and so here we have our camshaft which is opening and closing the valves it rotates one time for every two rotations of the crankshaft located below it and so as this camshaft is rotating which passes along through the center of this V it's causing these push rods to move up and down now having just a single camshaft of course is going to limit your control and so while in 2014 Chevy did introduce cam phasing on the Corvette with the lt1 engine they could only change the timing of the valves but they couldn't change them independently of one another so valve duration valve overlap between the exhaust and the intake valve and valve lift all remain the same the only thing they're changing is when that camshaft timing occurs so when its opening and closing the intake and exhaust valves relative to the rotation of the crankshaft now as you could imagine if you were to have two camshafts above the cylinders on each side you've got independent control of the intake and the exhaust valves and you can also use more profiles along that camshaft so you can change the valve lift and the valve duration for higher rpm in order to get better air flow at that higher rpm and you can also maximize the timing how much overlap you have between the intake and the exhaust valves to allow for the best the optimal airflow for any rpm range, so if you enjoyed watching this video I will include links to other relevant videos that you can check out that you may be interested in as always, if you have any questions or comments of course feel free to leave those below. Thanks for watching.

Channel: Engineering Explained

Views: 772,971

Keywords: pushrod, v8, v8 engine, engine revs, revving, 2 step, backfire, turbo, supercharger, high revving, low revving engine, american muscle, why low revving, low revs, low RPM, low redline, redline, engine, fast car, used car, buy car

Id: S9E2nCXNHU8

Length: 9min 21sec (561 seconds)

Published: Wed Apr 25 2018