# Combustion Engine Rotation **URL:** \ **Category:** Factual Questions\ **Created:** [May 22, 2017, 8:38pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233 "2017-05-22T20:38:22Z")\ **Posts on this page:** 7\ **Page:** 3
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [May 25, 2017, 7:20pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/41 "2017-05-25T19:20:10Z")
> [@Weisshund](#): > > It is a roller cam yes. > Gentle? no. > Put the head on a bench and run the cam at operational speeds. Just because you can hear the valve closing doesn’t mean anything. you can’t let valves snap closed uncontrolled because they will _stretch_, which is what leads to breakage. Plus it will accelerate wear of the valve/seat interface. if they close in a controlled manner by following a cam lobe down (whether closed via spring or via rocker arm) there’s no room for them to stretch. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 25, 2017, 7:35pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/42 "2017-05-25T19:35:29Z")
> [@Weisshund](#): > > From a crank/piston standpoint, what optimizations? This would be the only assembly changing what it is doing. > servo activated valves wont care which way we spin, nor will a hall sensor triggered ignition. > > Not saying you are wrong, i’m just curious Check out the list of stuff I laid out in post #26; there are many things changing what they are doing when you try to run an engine in the other direction. > [@Weisshund](#): > > It is a roller cam yes. > Gentle? no. > Put the head on a bench and run the cam at operational speeds. It certainly looks like it closes rapidly - except for the last little bit, the closing ramp. It’s like a [C-130 performing a tactical approach:](https://www.youtube.com/watch?v=ZT-toF6iXLU) it may drop like a mofo during final approach, but just like any other plane, it still has to flare at the last second for a gentle touchdown, lest bad things happen. About ten years ago I worked on a prototype hydraulically actuated valvetrain, and moderating the closing impact forces was an important consideration. It’s been a while, but ISTR a setup wherein during the last few mm of travel, the end of the valve stem entered a sleeve with tight clearances, such that hydraulic fluid being driven through those clearances at high velocities by the advancing valve stem generated braking forces that slowed the valve to an acceptable touchdown speed before the valve head actually contacted the seat. ---
**Author:** ![Weisshund](https://avatars.discourse-cdn.com/v4/letter/w/ba8739/32.png) [@Weisshund](https://boards.straightdope.com/u/Weisshund)\ **Post date:** [May 25, 2017, 10:09pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/43 "2017-05-25T22:09:27Z")
> [@Machine\_Elf](#): > > Check out the list of stuff I laid out in post #26; there are many things changing what they are doing when you try to run an engine in the other direction. Only crank assembly thing i noted in it is offset wrist pins on some engines. You would of course not use them and use a centered assembly. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [May 26, 2017, 5:17am UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/44 "2017-05-26T05:17:23Z")
I do know that it is entirely impossible for a wankel to run backwards. ---
**Author:** ![Fallsguy](https://avatars.discourse-cdn.com/v4/letter/f/bb73d2/32.png) [@Fallsguy](https://boards.straightdope.com/u/Fallsguy)\ **Post date:** [May 26, 2017, 8:54pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/45 "2017-05-26T20:54:00Z")
> [@Machine\_Elf](#): > > “Efficient”? In what sense of the word? A pair of 2-horsepower engines is bulkier, heavier, and more expensive than a single 4-horsepower engine. And probably less fuel-efficient, too. What benefit did you have in mind? I simply thought discharging grass on both sides might be more efficient than forcing it all out 1 side. And to the rest of the posters, thanks all, I learned quite a bit. ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [May 26, 2017, 10:20pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/46 "2017-05-26T22:20:11Z")
> [@Machine\_Elf](#): > > …ISTR a setup wherein during the last few mm of travel, the end of the valve stem entered a sleeve with tight clearances, such that hydraulic fluid being driven through those clearances at high velocities by the advancing valve stem generated braking forces that slowed the valve to an acceptable touchdown speed before the valve head actually contacted the seat. That’s a very common way to snub hydraulic actuators. Not surprising it was used on your prototype. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 27, 2017, 2:22am UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/47 "2017-05-27T02:22:04Z")
> [@eschereal](#): > > I do know that it is entirely impossible for a wankel to run backwards. What’s the problem? [Looking at this Wankel animation,](https://www.youtube.com/watch?v=K6oQqN0fpk8) it certainly appears that it would pump air backwards through the engine just like a conventional four-stroke piston engine; ISTM you’d just need to introduce fuel in the right place and tweak the spark timing, but those are the same issues you’d have with a piston engine. What am I missing? > [@Fallsguy](#): > > I simply thought discharging grass on both sides might be more efficient than forcing it all out 1 side. Firing the grass to one side is handy if you’re bagging it, and it’s also handy for mowing next to landscaping or parked cars, so that you can blow the clippings away from the bushes/flowers/cars instead of all over them. It would be easy to design a single-engine/blade mower with chutes going to both sides, but for those reasons I don’t think it would be a big seller. [Previous page](https://boards.straightdope.com/t/combustion-engine-rotation/787233.md?page=2)