# 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:** 20\ **Page:** 2
**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 24, 2017, 1:00pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/21 "2017-05-24T13:00:31Z")
> [@Weisshund](#): > > Um i do believe we were kind of talking about it, and it is in vein of what the OP asked about. Point taken, I’ll cop to being a bit narrowly focused on light duty engines. > [@](#): > > And there is the desmodromic valve train, which has been used for quite a while, which literally and solidly slams the valves down and forcefully holds them shut with the cam. > If you want to pop a valve head off, mis-adjusting that system is the way to go. > > And honestly, at 7000 rpms, do you think the valve lands very softly? that is _not_ what desmodromic valvetrains do; the valve still follows the cam profile down as it lands on the seat. Desmo valvetrains are meant to eliminate the possibility of valve loft/valve float at high RPM. ---
**Author:** ![Corry\_El](https://avatars.discourse-cdn.com/v4/letter/c/ecae2f/32.png) [@Corry\_El](https://boards.straightdope.com/u/Corry_El)\ **Post date:** [May 24, 2017, 1:31pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/22 "2017-05-24T13:31:17Z")
> [@electronbee](#): > > I’ll have to look into marine engine system, to see if the engine rotation can be reversed. That might have been the way, but, something tells me not so much anymore. Do look into it, because something is telling you wrong. 🙂 The dominant propulsion arrangement of large merchant ships is a low speed two stroke diesel directly connected to a fixed pitch propeller: you reverse the engine’s rotation to back down. That’s been true for many decades and if anything more true now. Controllable pitch props and diesel electric drive are more common now in big merchant ships, but still rare in ‘everyday’ big tankers, bulk carriers, container ships etc. The other arrangments are more common in passenger and other specialized vessels. And decades ago many big merchant ships were steam powered, whereas hardly any are now. So the overall % of ships propelled by direct connected direct reversing two stroke diesels is higher now. That’s not including smaller merchant ships which usually have four stroke diesels connected through reduction gears to controllable pitch propellers; and very small ships or workboats (tugs, towboats, in the US particularly) might have fixed pitch props and reduction gears with a reverse gear. But ships over say 10,000 gt, big tankers, bulk carriers, container ships you see driving by an anchorage or terminal, not cruise ships etc, almost all of them have direct connected direct reversing two stroke diesels. And they did back when all camshafts were mechanical. Electronically controlled ‘camshaft less’ two strokes are a fairly recent innovation. Plenty of mechanical camshaft two strokes are still around. To reverse rotation with a mechanical camshaft, some combination of the camshaft (rotate, slide laterally to a different set of cams) and/or the cam followers move. That’s something one can read a lot about, all the different ways that’s been done over the decades in various makes and designs of marine diesels. But it’s obviously workable, two or four stroke (though four stroke direct reversing marine diesels _are_ now rare, unlike two strokes). ---
**Author:** ![Tom\_Terrific](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tom_terrific/32/342_2.png) [@Tom\_Terrific](https://boards.straightdope.com/u/Tom_Terrific)\ **Post date:** [May 24, 2017, 2:15pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/23 "2017-05-24T14:15:28Z")
Getting away from the marine engines for a bit, Hot Rod magazine or Car Craft magazine, in the late 70’s or so had an article with Smokey Yunick who claimed that he had reversed the rotation on his Hudson Hornet nascar racers. He reversed the rotation because it would help plant the right rear tire on the corners. Supposedly he hadn’t even told his driver that one. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 24, 2017, 3:05pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/24 "2017-05-24T15:05:38Z")
Could you make a reversing four-stroke by swapping the intake for the exhaust? That is, put some sort of valve between the carburetor (yeah, I know) and the intake manifold, and between the exhaust manifold and the muffler. To reverse the engine, you stop it, set the valves so that the carburetor now feeds into (what was) the exhaust manifold, etc. So, what had been the exhaust stroke is now the intake stroke, the power stroke is now the compression stroke, and so on. You’d need a starter motor that could turn the engine both directions, and you’d need some means of changing the spark timing when you changed rotation. Anything else? It wouldn’t be optimal, but would it work at all? ---
**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 24, 2017, 3:30pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/25 "2017-05-24T15:30:32Z")
Really, in a 4-stroke, the cam is the thing that matters. If it always rotates in the same direction, it would not matter which way the crank is turning. The pistons just go up and down, doing whatever the valve cycle dictates. ---
**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 24, 2017, 4:49pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/26 "2017-05-24T16:49:56Z")
> [@Robot\_Arm](#): > > Could you make a reversing four-stroke by swapping the intake for the exhaust? That is, put some sort of valve between the carburetor (yeah, I know) and the intake manifold, and between the exhaust manifold and the muffler. To reverse the engine, you stop it, set the valves so that the carburetor now feeds into (what was) the exhaust manifold, etc. So, what had been the exhaust stroke is now the intake stroke, the power stroke is now the compression stroke, and so on. You’d need a starter motor that could turn the engine both directions, and you’d need some means of changing the spark timing when you changed rotation. Anything else? > > It wouldn’t be optimal, but would it work at all? Should work, but as you note, it won’t be optimal: [ul][li]intake manifolds are often made of aluminum or plastic, which won’t tolerate hot exhaust very well.[/li][li]Exhaust valves often have very deliberate cooling measures designed into them, including sometimes having a hollow interior filled with molten sodium that bounces around and helps carry heat up to the valve guide; intake valves generally rely on the passage of low-temperature fresh air/mixture to keep them cool. If you reverse the direction of air movement, you may find that using the intake valves as exhaust valves results in their getting hot enough to cause preignition, which can rapidly wreck the engine.[/li][li]Intake valves are generally larger than exhaust valves to promote better breathing. If the engine has three valves per cylinder, it’s usually two intake valves and one exhaust valve; it won’t breathe as well flowing the other way.[/li][li]Intake ports are typically shaped to set up a particular circulation pattern in the combustion chamber to facilitate fuel/air mixing and help speed up combustion. Exhaust ports aren’t shaped the same way, and so won’t produce those same flow patterns.[/li][li]Intake and exhaust ports are sometimes designed to take advantage of resonance to promote good breathing at high RPM. Temperature affects speed of sound, so if the exhaust ports are flowing cold fresh air and the intake ports are flowing hot exhaust, the tuning will be off; at high RPM, it won’t breathe as well as it did.[/li][\*]As the engine rolls through TDC, the connecting rod switches from pushing the piston against one side of the bore to pushing it against the other side of the bore. Engines are often designed with the wrist pin offset slightly to one side of the piston so that this changeover happens slightly before TDC, before ignition/combustion has a chance to build up a lot of pressure. If the engine is rotating the other way, the changeover will happen _after_ TDC, when combustion pressure has increased considerably; the piston will slap harder against the other side of the bore than it previously did, increasing noise and wear. [/ul] ---
**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 24, 2017, 4:54pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/27 "2017-05-24T16:54:22Z")
And then there is the Miller cycle engine, which offsets the crank slightly, to make the power stroke longer than the compression stroke. If you tried to run one of those backwards, the performance would really suck. ---
**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 24, 2017, 5:00pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/28 "2017-05-24T17:00:39Z")
> [@eschereal](#): > > And then there is the Miller cycle engine, which offsets the crank slightly, to make the power stroke longer than the compression stroke. If you tried to run one of those backwards, the performance would really suck. that’s not what a “Miller cycle” engine does. it uses late intake valve closing to make the effective expansion ratio greater than the compression ratio, combined with a supercharger to make up for the loss of static compression. hybrids use a similar trick to simulate an Atkinson cycle engine, without the supercharger. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 24, 2017, 5:12pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/29 "2017-05-24T17:12:39Z")
I’ve heard that in the early days of single-propeller airplanes (like the ones used in WWI), the plane would perform differently turning left vs. right because of the angular momentum of the engine. To stay unpredictable, fighter pilots would have two different propellers they could swap out, and would reverse the direction of the engine as appropriate. ---
**Author:** ![Corry\_El](https://avatars.discourse-cdn.com/v4/letter/c/ecae2f/32.png) [@Corry\_El](https://boards.straightdope.com/u/Corry_El)\ **Post date:** [May 24, 2017, 6:43pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/30 "2017-05-24T18:43:38Z")
> [@Chronos](#): > > I’ve heard that in the early days of single-propeller airplanes (like the ones used in WWI), the plane would perform differently turning left vs. right because of the angular momentum of the engine. To stay unpredictable, fighter pilots would have two different propellers they could swap out, and would reverse the direction of the engine as appropriate. Some WWI aircraft had so called rotary engines, not in the modern sense of a Wankel engine, but a type of radial piston engine where the single pin crankshaft stood still attached to the plane and the engine rotated with the propeller, rather than engine staying still and crankshaft rotating with the propeller. Thus there was a relatively notable gyroscopic precession effect in turns. However AFAIK all the major engines of that type were right handed, ie one mounted in the nose would turn clockwise from the pilot’s POV. Thus the effect on turns was always a nose down moment in a right turn and a nose up moment in a left turn, tending to ease the former (naturally diving turn) and slow down the latter. Being able to reverse the engine rotation and switch propellers (from right to left handed) sounds like what somebody in latter days would suggest they should have done rather than something that actually happened. And the perceived importance of this is also based on a tendency to evaluate the issue in terms of one a/c, the Sopwith Camel, and stories of it perhaps embellished for effect. The linked article measured the effect on a reproduction Camel and found it significant but not dramatic. And some other rotary engine fighters, even other Sopwith fighters, didn’t have reputations for being much trickier to handle than non-rotaries, WWI fighters were generally unsafe by any later standard. [http://www.flyingmag.com/pilots-places/pilots-adventures-more/calculated-sopwith-camel](http://www.flyingmag.com/pilots-places/pilots-adventures-more/calculated-sopwith-camel) ---
**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 24, 2017, 6:56pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/31 "2017-05-24T18:56:26Z")
> [@Corry\_El](#): > > Some WWI aircraft had so called rotary engines, not in the modern sense of a Wankel engine, but a type of radial piston engine where the single pin crankshaft stood still attached to the plane and the engine rotated with the propeller, rather than engine staying still and crankshaft rotating with the propeller. Thus there was a relatively notable gyroscopic precession effect in turns. animation of such an engine: > **[Animated Engines - Gnome Rotary](https://animatedengines.com/gnome.html)** > > Animated illustration and description of the Gnome rotary engine. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 24, 2017, 8:24pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/32 "2017-05-24T20:24:44Z")
[quote=“Machine\_Elf, post:26, topic:787233”] Should work, but as you note, it won’t be optimal: [ul][li]intake manifolds are often made of aluminum or plastic, which won’t tolerate hot exhaust very well.[/li][li]Exhaust valves often have very deliberate cooling measures designed into them, including sometimes having a hollow interior filled with molten sodium that bounces around and helps carry heat up to the valve guide; intake valves generally rely on the passage of low-temperature fresh air/mixture to keep them cool. If you reverse the direction of air movement, you may find that using the intake valves as exhaust valves results in their getting hot enough to cause preignition, which can rapidly wreck the engine.[/li][li]Intake valves are generally larger than exhaust valves to promote better breathing. If the engine has three valves per cylinder, it’s usually two intake valves and one exhaust valve; it won’t breathe as well flowing the other way.[/li][li]Intake ports are typically shaped to set up a particular circulation pattern in the combustion chamber to facilitate fuel/air mixing and help speed up combustion. Exhaust ports aren’t shaped the same way, and so won’t produce those same flow patterns.[/li][li]Intake and exhaust ports are sometimes designed to take advantage of resonance to promote good breathing at high RPM. Temperature affects speed of sound, so if the exhaust ports are flowing cold fresh air and the intake ports are flowing hot exhaust, the tuning will be off; at high RPM, it won’t breathe as well as it did.[/li][li]As the engine rolls through TDC, the connecting rod switches from pushing the piston against one side of the bore to pushing it against the other side of the bore. Engines are often designed with the wrist pin offset slightly to one side of the piston so that this changeover happens slightly before TDC, before ignition/combustion has a chance to build up a lot of pressure. If the engine is rotating the other way, the changeover will happen _after_ TDC, when combustion pressure has increased considerably; the piston will slap harder against the other side of the bore than it previously did, increasing noise and wear. [/ul][/li][/QUOTE] Very interesting post. Just the sort of thing I come to the board for. The last engine I did any kind of work on was from the 1960s, and the tech was probably decades older than that; plastic intake manifolds and offset wrist pins are well beyond my experience. Many thanks. Still, it sounds like some of those things could be overcome. Have all the valves filled with sodium; you’d only need the extra heat transfer on whichever valves would currently be the exhaust, but it doesn’t seem like it would hurt the intake valves. I don’t know why anyone would ever need a reversing four-stroke, but I’m surprised it seems possible at all. ---
**Author:** ![busterpickle](https://avatars.discourse-cdn.com/v4/letter/b/258eb7/32.png) [@busterpickle](https://boards.straightdope.com/u/busterpickle)\ **Post date:** [May 24, 2017, 8:32pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/33 "2017-05-24T20:32:48Z")
> [@jz78817](#): > > that’s not what a “Miller cycle” engine does. it uses late intake valve closing to make the effective expansion ratio greater than the compression ratio, combined with a supercharger to make up for the loss of static compression. > > hybrids use a similar trick to simulate an Atkinson cycle engine, without the supercharger. So, is that VTEC and other make similar tech, where they manipulate the intake/exhaust valve profile depending on engine RPM? ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 24, 2017, 8:35pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/34 "2017-05-24T20:35:02Z")
> [@jz78817](#): > > animation of such an engine: > > [Animated Engines - Gnome Rotary](http://www.animatedengines.com/gnome.html) When I first heard about rotary engines they sounded pretty bonkers to me, too. The text accompanying that animation explains most of the issues. I still wonder about how the fuel intake worked (it sounds like not all rotaries had the valve in the piston like the Gnome did) and how power was controlled (I’ve heard the pilot just cut the ignition on and off, causing it to sound like a barking dog, hence “dogfighting”). ---
**Author:** ![Weisshund](https://avatars.discourse-cdn.com/v4/letter/w/ba8739/32.png) [@Weisshund](https://boards.straightdope.com/u/Weisshund)\ **Post date:** [May 24, 2017, 9:45pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/35 "2017-05-24T21:45:05Z")
> [@jz78817](#): > > that is _not_ what desmodromic valvetrains do; the valve still follows the cam profile down as it lands on the seat. Desmo valvetrains are meant to eliminate the possibility of valve loft/valve float at high RPM. Yes, and down very fast because things using that valve system tend to be high performance with aggressive profiles and high RPMs , and the valve comes down and is held forcefully shut by solid mechanical force as opposed to forgiving spring. 2/1000ths too tight and SNAP, off with his head! or worse, if it can get any worse. [Cam Profile with aggressive asymmetrical closing ramp](http://img.bikebros.co.jp/vb_img/tieup/sports/yoshimura/08/img/detail01_b.jpg) You will notice that is cut to pretty much drop it shut. Spin that at 3500 rpms (the cam itself) and tell me if you do not both hear and feel the valves closing? Now imagine it in the Ducati spinning much faster. You are saying servo activated valves, which are already in use, are going to be more detrimental than that already is? ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [May 24, 2017, 11:12pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/36 "2017-05-24T23:12:24Z")
Bought a non-running used 1976 Yamaha 650 vertical twin a while back. Had compression, spark and fuel. It should at LEAST pop. Took me too long to figure out that the previous owner had rebuilt the engine 180 degrees off (cam effectively rotated 180 degrees off when cam chain was put on). I switched the spark plug wires from left to right, right to left and it ran. But poorly. Spark advance was all screwed up. ---
**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 24, 2017, 11:31pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/37 "2017-05-24T23:31:07Z")
> [@electronbee](#): > > So, is that VTEC and other make similar tech, where they manipulate the intake/exhaust valve profile depending on engine RPM? No. VTEC is a method to optimize intake valve timing and lift for various engine speeds. it’s still an Otto cycle engine; the expansion ratio is 1.0. ---
**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, 1:26pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/38 "2017-05-25T13:26:42Z")
> [@Weisshund](#): > > [Cam Profile with aggressive asymmetrical closing ramp](http://img.bikebros.co.jp/vb_img/tieup/sports/yoshimura/08/img/detail01_b.jpg) > You will notice that is cut to pretty much drop it shut. The cam lob on the left is clearly asymmetric, yes, but if it’s working with a roller-follower (I suspect it is, [based on the shape](http://www.crankshaftcoalition.com/wiki/How_to_choose_a_camshaft#Flat_tappet_vs._roller)), then it may still be proving a gentle touchdown. > [@Robot Arm](#): > > Still, it sounds like some of those things could be overcome. Have all the valves filled with sodium; you’d only need the extra heat transfer on whichever valves would currently be the exhaust, but it doesn’t seem like it would hurt the intake valves. Sure, you could overcome all those things, providing the flexibility to run the engine in either direction, but then of course it will no longer be optimized for one particular direction; it’s just a matter of locking in some choices during the design/fabrication phase. ---
**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, 6:30pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/39 "2017-05-25T18:30:19Z")
> [@Machine\_Elf](#): > > The cam lob on the left is clearly asymmetric, yes, but if it’s working with a roller-follower It is a roller cam yes. Gentle? no. Put the head on a bench and run the cam at operational speeds. Anyways, the argument about the valves snapping apart is pointless. There are already engines with such activated valves in commercial use. And Freevalve is already demonstrating them in the smaller engined automotive market. ---
**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, 6:58pm UTC](https://boards.straightdope.com/t/combustion-engine-rotation/787233/40 "2017-05-25T18:58:29Z")
> [@](#): > > Sure, you could overcome all those things, providing the flexibility to run the engine in either direction, but then of course it will no longer be optimized for one particular direction; it’s just a matter of locking in some choices during the design/fabrication phase. 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 [Previous page](https://boards.straightdope.com/t/combustion-engine-rotation/787233.md?page=1) [Next page](https://boards.straightdope.com/t/combustion-engine-rotation/787233.md?page=3)