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.