# Opposed piston diesel engine **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2010, 6:07pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424 "2010-07-12T18:07:43Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![tropes](https://avatars.discourse-cdn.com/v4/letter/t/e5b9ba/32.png) [@tropes](https://boards.straightdope.com/u/tropes)\ **Post date:** [July 12, 2010, 6:07pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/1 "2010-07-12T18:07:43Z")
I have studied many pictures of the Commer Diesel engine but I can’t figure out how the pistons can travel in the same cylinder yet the crankshaft rods are offset on the crank. I assume the rockers offset the two rods or the piston rods are not in the center of the pistons. Does anyone know? tropes ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 12, 2010, 6:29pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/2 "2010-07-12T18:29:32Z")
The diagrams I’ve seen seem to show the rockers are offset from the center of the pistons and connected to the rods with a simple side by side connection. Those are diagrams though, not construction plans. But it shouldn’t matter how the offset is constructed. There will be some additional stress on the bearings in the mechanism, but overall should be generally balanced by the two pistons operating in unison. It probably adds some degree of vibration and wear in the whole crankshaft, rod, and rocker mechanism. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 12, 2010, 6:36pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/3 "2010-07-12T18:36:38Z")
Never mind. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [July 12, 2010, 6:43pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/4 "2010-07-12T18:43:29Z")
linked example to look at? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [July 12, 2010, 6:47pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/5 "2010-07-12T18:47:29Z")
> [@Magiver](#): > > linked example to look at? [http://www.oldengine.org/members/diesel/Rootes-ListerTS3/TS36.jpg](http://www.oldengine.org/members/diesel/Rootes-ListerTS3/TS36.jpg) Is there something specific you’re not getting. In my mind, it doesn’t seem like there would be a problem with this setup. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [July 12, 2010, 6:51pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/6 "2010-07-12T18:51:00Z")
[http://www.sa.hillman.org.au/TS3.htm](http://www.sa.hillman.org.au/TS3.htm) See especially animation at bottom. Nice way of getting good end-end scavenge without requiring an operable valve for exhaust. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [July 12, 2010, 7:02pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/7 "2010-07-12T19:02:26Z")
I don’t think it should be a problem having the rockers angled to allow the pistons and camshaft throws to be offset. For example, take a look at these VW valve rockers.[http://www.vdubn.com/sandrailing/engine/vlvetrn1.jpg](http://www.vdubn.com/sandrailing/engine/vlvetrn1.jpg) ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [July 12, 2010, 8:11pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/8 "2010-07-12T20:11:22Z")
Interesting design but if you applied the ideas behind it you could make a diesel out of a rotary engine using a roots blower to raise cylinder pressure. ---
**Author:** ![tropes](https://avatars.discourse-cdn.com/v4/letter/t/e5b9ba/32.png) [@tropes](https://boards.straightdope.com/u/tropes)\ **Post date:** [July 12, 2010, 10:06pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/9 "2010-07-12T22:06:45Z")
> [@Joey\_P](#): > > [http://www.oldengine.org/members/diesel/Rootes-ListerTS3/TS36.jpg](http://www.oldengine.org/members/diesel/Rootes-ListerTS3/TS36.jpg) > > Is there something specific you’re not getting. In my mind, it doesn’t seem like there would be a problem with this setup. Not a problem; just wondering how the offset is achieved on the crankshaft. The center of the piston and the center of the crank rod are not on the same plane. It must be either with the rockers or the piston rods. Tropes ---
**Author:** ![tropes](https://avatars.discourse-cdn.com/v4/letter/t/e5b9ba/32.png) [@tropes](https://boards.straightdope.com/u/tropes)\ **Post date:** [July 12, 2010, 10:20pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/10 "2010-07-12T22:20:03Z")
> [@74westy](#): > > I don’t think it should be a problem having the rockers angled to allow the pistons and camshaft throws to be offset. For example, take a look at these VW valve rockers.[http://www.vdubn.com/sandrailing/engine/vlvetrn1.jpg](http://www.vdubn.com/sandrailing/engine/vlvetrn1.jpg) The rockers are not angled but the rods (piston rod and crank rod) might be attached on opposite sides of the rocker. Tropes ---
**Author:** ![retterath](https://avatars.discourse-cdn.com/v4/letter/r/919ad9/32.png) [@retterath](https://boards.straightdope.com/u/retterath)\ **Post date:** [July 12, 2010, 10:25pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/11 "2010-07-12T22:25:19Z")
I don’t know about this particular engine, but harley’s have connecting rods that fit one inside the other, so no offset is required. [http://img819.imageshack.us/img819/928/harrod.jpg](http://img819.imageshack.us/i/harrod.jpg/) ---
**Author:** ![tropes](https://avatars.discourse-cdn.com/v4/letter/t/e5b9ba/32.png) [@tropes](https://boards.straightdope.com/u/tropes)\ **Post date:** [July 12, 2010, 11:38pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/12 "2010-07-12T23:38:25Z")
> [@retterath](#): > > I don’t know about this particular engine, but harley’s have connecting rods that fit one inside the other, so no offset is required. > > [http://img819.imageshack.us/img819/928/harrod.jpg](http://img819.imageshack.us/i/harrod.jpg/) The Commer has a separate journal for each rod but I find this Harley rod idea very interesting. I may try this on my next Sotropa Motor [http://www.youtube.com/watch?v=N8Eq4D2c7VQ&NR=1](http://www.youtube.com/watch?v=N8Eq4D2c7VQ&NR=1) Tropes ---
**Author:** ![CutterJohn](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@CutterJohn](https://boards.straightdope.com/u/CutterJohn)\ **Post date:** [July 13, 2010, 2:34am UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/13 "2010-07-13T02:34:53Z")
or one side could be a fork and the other a single rod, so that no offset is needed. ---
**Author:** ![retterath](https://avatars.discourse-cdn.com/v4/letter/r/919ad9/32.png) [@retterath](https://boards.straightdope.com/u/retterath)\ **Post date:** [July 13, 2010, 2:55am UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/14 "2010-07-13T02:55:08Z")
see post #11 😉 ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [July 13, 2010, 4:43pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/15 "2010-07-13T16:43:39Z")
> [@Kevbo](#): > > [http://www.sa.hillman.org.au/TS3.htm](http://www.sa.hillman.org.au/TS3.htm) > See especially animation at bottom. > > Nice way of getting good end-end scavenge without requiring an operable valve for exhaust. I think I remember an interesting feature about this design; variable compression ratio. The engine is at idle and the turbo is producine minimal boost, the comp.ratio os something like 22:1. At power, with the turbo at full boost, the rocker arm (somehow :rolleyes:) reduces comp. ratio to 18:1. Any industrial engineers confirm this? ---
**Author:** ![tropes](https://avatars.discourse-cdn.com/v4/letter/t/e5b9ba/32.png) [@tropes](https://boards.straightdope.com/u/tropes)\ **Post date:** [July 13, 2010, 6:02pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/16 "2010-07-13T18:02:16Z")
> [@Enola\_Straight](#): > > I think I remember an interesting feature about this design; variable compression ratio. > > The engine is at idle and the turbo is producine minimal boost, the comp.ratio os something like 22:1. > > At power, with the turbo at full boost, the rocker arm (somehow :rolleyes:) reduces comp. ratio to 18:1. > > Any industrial engineers confirm this? Because the rocker arm is fixed in position, I see no way it can affect the compression ratio. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [July 13, 2010, 6:11pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/17 "2010-07-13T18:11:31Z")
Wow…I just totally figured out what this whole thread is about. I think. It just hadn’t clicked earlier on. Could the connecting rods be offset like this [wrench?](http://buy1.snapon.com/catalog/OBJECTS/48800/48726.JPG) ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [July 13, 2010, 6:25pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/18 "2010-07-13T18:25:24Z")
> [@retterath](#): > > I don’t know about this particular engine, but harley’s have connecting rods that fit one inside the other, so no offset is required. > > [http://img819.imageshack.us/img819/928/harrod.jpg](http://img819.imageshack.us/i/harrod.jpg/) I bet that sounds awesome idling but Kevbo’s [diagram](http://www.sa.hillman.org.au/TS3.htm) shows the two pistons being on different crank throws. they are on opposite sides of the crankshaft but are both “up” at the same time. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [July 13, 2010, 6:29pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/19 "2010-07-13T18:29:34Z")
> [@tropes](#): > > The rockers are not angled but the rods (piston rod and crank rod) might be attached on opposite sides of the rocker. > Tropes I like my design better. 🙂 Since the rockers are on opposite sides they would all be the identical (angled) part. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [July 13, 2010, 11:44pm UTC](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424/20 "2010-07-13T23:44:07Z")
> [@Enola\_Straight](#): > > I think I remember an interesting feature about this design; variable compression ratio. > > The engine is at idle and the turbo is producine minimal boost, the comp.ratio os something like 22:1. > > At power, with the turbo at full boost, the rocker arm (somehow :rolleyes:) reduces comp. ratio to 18:1. > > Any industrial engineers confirm this? You remember right, but not with the Commer engine. The engine in question is the Rolls Royce K60, which you can Google but it’s hard to find much real information on it other than in engineering message boards. It was (and possibly still is) used for military vehicles and can accept a wide variety of fuels. > [@tropes](#): > > Because the rocker arm is fixed in position, I see no way it can affect the compression ratio. The K60 has two seperate crankshafts on either side (actually top and bottom - the cylinders are vertical) and the phase angle between them can be varied. Check out other twin-crank designs here: [http://www.histomobile.com/dvd\_histomobile/usa/tech/121-2.asp](http://www.histomobile.com/dvd_histomobile/usa/tech/121-2.asp) [Next page](https://boards.straightdope.com/t/opposed-piston-diesel-engine/546424.md?page=2)