The Challenges of Pushrod Engines: Understanding the Limitations of High-RPM Performance
One of the primary reasons why pushrod engines struggle to rev high is due to the reciprocating mass that comes with this design. Unlike overhead camshafts, which have a more compact and rigid design, pushrod engines feature long rods that connect the piston to the valve train. This results in a significant amount of mass that oscillates back and forth as the engine operates, creating a challenge for maintaining contact with the cam profile at high RPMs. As a result, valve float becomes a major issue, making it difficult to maintain consistent engine performance.
To address this problem, engineers have developed various strategies, such as using stiffer springs. While these springs can help maintain contact with the cam profile, they also introduce additional losses due to increased work required to compress them. This efficiency loss can significantly impact engine performance at high RPMs. In contrast, overhead camshaft engines do not suffer from this same issue, as their design allows for more precise control over valve timing and lift.
In NASCAR engines, stiffer springs are often used in conjunction with pushrods to maintain contact with the cam profile at high RPMs. However, even with these modifications, engines typically rev up to around 10,000 RPM before reaching their performance limits. In contrast, Formula One engines can reach speeds of over 20,000 RPM due to their use of air springs. These pneumatic valves provide a more precise and controlled valve train, allowing for better airflow at high RPMs.
The use of air springs also enables engineers to take advantage of advanced camshaft designs, which can optimize valve timing and lift for specific RPM ranges. This allows for improved airflow and increased engine performance, even at extremely high RPMs. In contrast, pushrod engines are often limited by their design, which can restrict airflow at high RPMs.
Another challenge facing pushrod engines is air flow. At low RPMs, having fewer valves per cylinder can actually be beneficial, as it allows for better fuel mixing and increased velocity of the intake charge. However, as engine RPM increases, more air must be drawn into the engine to maintain performance. This means that a four-valve system is often preferred over a two-valve system, as it provides more airflow at high RPMs.
To mitigate this challenge, engineers have developed various cylinder head designs that allow for more airflow at high RPMs. These designs often feature valves that come down further to increase the area available for airflow. However, even with these modifications, pushrod engines still struggle to match the performance of overhead camshaft engines at extremely high RPMs.
A notable example of a pushrod engine is the 3D printed Chevy LS3 V8. This engine features a unique design that showcases some of the challenges and limitations of pushrod engines. As the camshaft rotates, it passes through the center of the valve train, causing the pushrods to move up and down. While this design allows for more precise control over valve timing and lift, it also introduces some inherent challenges.
One major limitation of pushrod engines is the single camshaft design, which restricts independent control of the intake and exhaust valves. This means that engineers are limited in their ability to adjust valve timing and lift for specific RPM ranges. However, this limitation can be addressed by using two camshafts, one above each cylinder. This allows for independent control of the intake and exhaust valves, as well as more complex camshaft profiles that can optimize valve lift and duration for high RPMs.
By optimizing valve timing and lift, pushrod engines can improve airflow at high RPMs. Additionally, maximizing overlap between the intake and exhaust valves can also help to improve engine performance. By carefully adjusting these parameters, engineers can create a pushrod engine that can match the performance of overhead camshaft engines at extremely high RPMs.
In conclusion, pushrod engines face several challenges when it comes to high-RPM performance. From reciprocating mass and valve float to air flow limitations and single camshaft designs, each of these factors contributes to the difficulties of maintaining consistent engine performance at extreme RPMs. However, by understanding these challenges and using innovative design solutions, engineers can push the limits of what is possible with pushrod engines.
"WEBVTTKind: captionsLanguage: enHello everyone and welcome in this videowe'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 adisassembled single cylinder pushrod engine as well as a 3D printed. Chevy LS3 V8pushrod engine okayso before we get into why these engines don't tend to rev very high we need to understand how they operateso here we have our piston which of course is going to be moving up and down forcing this crankshaftwhich it's attached to to rotate so the crankshaft is ultimately what's going to be sending power to yourtransmission so this piston is moving up and down and as it moves up and down it forces this crankshaft to rotatenow geared to the crankshaft is the camshaft so it's geared at a two-to-one ratio so for every tworotations of the crankshaft this camshaft will rotate once so you'll see that's geared thereand as that crankshaft rotates it forces this camshaft to rotate which you can see has these lobes on itwhich are what are going to be opening and closing the valves, so you can see the lobes on the camshaftand then we have a cam followeryou could also call it a valve lifteror a tappet which will be following that profile of the camshaftso as that camshaft is rotating it will force this lifter upyou can see as it follows that profile and then as it continues to rotatethat lifter will come back down with it, so it pushes it up when it reaches the peak of that lobeand 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 andas that camshaft rotates the tappet will force the push rod to move up and down the push rod then pushes up against the rockerarm which forces the valve to open so you can see that this rocker arm will get pushed up, and as it pushes upit's attached to the valve on the other end and so as it pushes up itforces that valve to move down so looking at the whole interaction you have your camshaft rotating that presses on the tappetwhich forces the push rod up the push rod forces the rocker arm to rotate and then as that rocker arm rotates itforces your valve down allowing for the airflow to either go in or out so now looking at the cylinder head herewe have the push, and you can see that as you push. This push rod down itforces that valve to openso you can see the valve opening right there as that push rod is moved and that's because it's rotating this rocker armhere 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 reasonwhy push rod engines don't tend to rev very high and that is valve float and so what happens is once you start getting intothose higher rpms that means this push rod this entire assembly is having to move back and forth very quicklyso these are reciprocating mass rather thanrotational mass so it starts to go back and forth very quickly and you have all of this mass within the systemthat's trying to change directions very quickly as you get into high rpm nowthe only thing that's keeping this valve from following the cam profile is this spring right hereso this springforces the valve to come back up once you go back to the flat portion of the cam shaft when you're not on that lobethat extends out and so this spring is what's doing all that work to make sure that this entire systemremains in contact with this cam lobebut as you get into the higher rpm thisreciprocating mass the spring doesn't tend to keep up with it and so as a result you'll have moments where no longer isyour tappet following that cam profileperfectly because this spring isn't keeping up with what you're demanding and so this push rod this tappet all of thisthe valve itself has weightand it's of coursereciprocating mass and so all of that mass istrying to go this way, and then the spring is trying to push it back, and if it doesn't maintain contactand it's not able to push it back quickly enough that means as this rotates aroundyou lose contact with the cam profileand there's a moment there where your valve opening is notrelated to your cam profile, and that's called valve float because you're no longer controlling your valve lift and durationusing the camshaft nowit'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 thingsjust don't work out in your favorbut worst case you could have this valve thencontacting the piston, which is on its way upand so you could have some severe engine damage if that were to occurbut overall you're not controlling the airflow the way the engine was designedto and as a result you're going to lose efficiency and at higher rpmit's not as easy to get that air flow and so ultimatelyvalve float because of all of this reciprocating mass which you don't have in other style engines such as overhead camshaftsyou're going to have run into that barrierwhere valve float is a very real challenge to deal with now you of course can use stiffer springs that can help you with maintainingcontact withthat cam profile up that of course is an efficiency loss because it takes more work then to compress that spring you're going to haveadditional losses using a stiffer spring so you may say okay well NASCAR has engines that are using push rodsand they're revving to 10,000 rpm, which is indeed true and of course using very stiff springsbut of course in Formula One you're seeing you know rpms as high as20,000 rpm and that's a result they're actually using air spring so rather than a steel coil springthey're using pneumatic springs using air pressure to force that valve up which is easier to controlrather than using a steel spring which starts to vibrate, and then you start losing contact with this cam profile nowthat's of course also due to regulation so you should always take you know rpm range limits in racing with a grain of salt becauseyou know they're restricted within the rules that they can be in so NASCAR could perhaps Rev higher, Formula One could perhaps Rev higherbut there are limits set in place that limit them to those peak rpms, but generally speakingdual overhead cam solutions using pneumatic valves are going to be able to Rev quite a bit higher than using a push rod setupwhich 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 flowbecause at high rpm you need lots of air and while it ispossible for a pushrod engine to use more than just two valves per cylinderit's quite a complex design and so most often you will see two valves per cylinder in pushrod enginesand so what this limits it is its airflow at higher rpm so at low RPMthere's actually nothing wrong you can benefit from havingless valves because it's going to increase the velocity of that intake charge coming in you'll have better fuel mixingbut as you get into higher rpm you of course need more air and engine revving at 10,000 rpmversus 5,000 rpmideally is going to be pulling in about twice as much air because it's revving twice as fast and has twice as manycombustion cycles occurring and so by increasing the number ofvalves you increase the area at which air can flow into the engine and thus how much air you can pack into it in thatshort duration when you're revving it super high rpm nowpushrod engines will often try to mitigate this with the cylinder head design and use valves that come down quite a bit more to allowfor more airflowbut ultimately a four valve system is going to be able to flow more air through it at higher rpmversus a two valve system, okay, so looking at this 3d printed Chevy ls3 v8pushrod enginewe can get into our final reason why they don't tend to read very high and that comes down to valve timing and so herewe have our camshaftwhich is opening and closing the valves it rotates one time for every two rotations of the crankshaftlocated below it and so as this camshaft is rotating which passes along through the center of this Vit's causing these push rods to move up and down now having just a single camshaftof course is going to limit your control and so while in2014 Chevy did introduce cam phasing on the Corvette with the lt1 enginethey could only change the timing of the valvesbut they couldn't change them independently of one anotherso valve duration valve overlap betweenthe exhaust and the intake valve and valve lift all remain the same the only thing they're changing iswhen that camshaft timing occurs so when its opening and closing the intake and exhaust valvesrelative to the rotation of the crankshaft now as you could imagine if you were to have two camshaftsabove the cylinders on each side you've got independent control of the intake and the exhaustvalves and you can also use more profiles along that camshaftso you can change the valve lift and the valve duration for higher rpm in order to get better air flow at that higherrpm and you can also maximizethe timing how much overlap you have between the intake and the exhaustvalves to allow for the best the optimal airflow for any rpm range, so if you enjoyed watching this videoI will include links to other relevant videos that you can check out that you may be interested in asalways, if you have any questions or comments of course feel free to leave those below. Thanks for watching.\n"
