Why not have 2-stroke Diesel with exhaust port intake valve?

[JimFife]

Two-Stroke diesels typically have exhaust valves, using their pistons to uncover intake ports. Why not reverse that, with exhaust ports and intake valves?
In the standard design, doesn’t the exhaust valve have to open too early in the power stroke, before the gas is fully expanded, to let the exhaust start going out before the intake port gets uncovered? Wouldn’t you get more power if the piston went further down, uncovering exhaust ports, before an intake valve opened in the head? Wouldn’t you also gain exhaust efficiency because the momentum of the gas is downward in the power stroke, helping push it out of exhaust ports?
This arrangement would also allow engine designs with variable (intake) valve timing. If you close the intake later, the mechanical compression is delayed; the compression ratio becomes smaller relative to the expansion ratio, extracting more work and reducing exhaust pressure, effectively creating an Atkinson cycle. A digital engine control could employ the variable timing at low and medium power for max efficiency but phase it out for maximum power situations.

 

[WolfHR]

My initial thought was that slow-revving diesels would need awkwardly long exhausts for exhaust-tuning (to return escaped air), and moreover could prove to be a bit of problem anyway if the engine was, as is the wont with diesels, turbo- or supercharged... Not that I'm an expert in the field, so I might be completely off the base.

 

[rmw]

Some of the action of the exhaust (in the exhaust ported two stroke diesels I am familiar with) acts to scavenge the cylinder aiding the air ingress.

 

[JimFife]

To RMW: ?THERE ARE 2 stroke diesels with exhaust ports & Intake valves? In my searching (admittedly biased toward Google images) its always the opposite. If "my" arrangement is valid and in use, do you know of any brands/models you could name for me?

 

[moon161]

I think the 2 strokers are usually turbo or supercharged, to boost cycle and or volumetric efficiency. Opposite of an atkinson cycle, in a way.

 

[BigClive]

Jim - I think your question has been answered to some extent here:

http://forums.autosport.com/index.php?showtopic=164465&pid=5647194&st=40&#entry5647194

Post 72 onwards especially.
I suspect the reasons given here of lubricating problems etc. are probably correct. It would not be too difficult to rearrange a Detroit Diesel to operate the way you suggest so I would think that GM have probably thoroughly tested your idea and found that their way is better.
Having written this - 2-stroke petrol engines seem to survive OK with cylinder exhaust ports and their exhaust is probably hotter than a diesel's exhaust.

 

[rmw]

BigClive,

Your post made me chuckle. I have quite a bit of experience with DD 2-cycle diesels, and have on more than one occasion seen them run backwards. So I guess it was doing exactly what JimFife was asking if could be done.

It didn't run well, by the way, because there was no 'blower' on the exhaust end to push the air through it, but it guaranteed ran - enough to trash the valve deck in the A/C compressor connected to the engine.

rmw

 

[BigClive]

I was going to write that it was interesting that the engine would run backwards without the help of the blower - but I imagine that the blower itself was running backwards drawing the exhaust out and the intake air in.

 

[rmw]

BigClive,

I guess that is the way it had to have gone down. I can tell you that the first time it ever happened it was a real mind bender to look at the side of a Greyhound sized bus and see black exhaust coming out of the air intake port. The eyes are saying "I see this" but the mind is saying "this doesn't compute". Then you remember having heard some old geezer (this happened before I became a geezer) making some kind of statement to the effect that Detriots would run either direction.

The next two or three times it happened, it was just a sick feeling in the pit of the stomach knowing that the A/C Compressor was about to get another new valve deck. Six cylinder A/C compressors seem to hate running backwards. Maybe they should have been designed with ports in the cylinder walls so that they could run either way too.

rmw

 

[patprimmer]

Or a ratchet in the drive pulley so it can only drive in one direction

Regards
Pat
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[LadaTrouble]

The Commer TS3 diesel had a dephaser in the injector pump drive so it wouldn't run backwards....although I've heard they would reverse direction if idled too low. The TS3 and Deltic engines had ported exhausts.

 

[PEW]

I used to ride a BSA C15 four stroke to college (well, forty years ago I did) and often saw an old boy on an ancient but immaculately kept Francis Barnett 2 stroke (complete with tall windscreen and steel leg guards). A classic "Frantic Barnyard", as we used to call them.

We used to stop at a set of traffic lights. One morning he got to the lights just in front of me. His engine was backfiring and chuffing out a lot more blue smoke than normal. It stalled at the lights and I was then stuck behind him as I wanted to turn left; it was a narrow road and the straight on traffic to our right prevented me getting round him.

After a couple of minutes of frantic kick-starting he managed to fire up his engine again but it was obvious something was wrong. It really wasn't sounding right and he was immediately surrounded by a big cloud of blue smoke. I was only a couple of yards behind him as he knocked it into gear and let the clutch out. The bike shot sharply backwards, his rear mudguard hitting my bike's front wheel and then dumping him on the pavement alongside my bike.

Somehow he'd started the engine backwards (I can only assume the timing was way out)! I helped pick him up but I have to admit, I was giggling the whole time, which he didn't appreciate one bit!

 

[LadaTrouble]

That happened to me on my BSA Bantam as a 16 year old apprentice. Starting my bike after the late night school class I let out the clutch and it shot backwards under the building behind me. I looked around and nobody had noticed, so started the bike and rode off again. I was very puzzled, it was like I crossed through a vortex into another world where things went backwards....I asked the mechanics at work and they said it was impossible for an engine to run backwards. I kept quiet about it for many years until I figured it out.

 

[BigClive]

Some model plane two-stroke engines are very prone to starting and running in the wrong direction. Some are so prone to this that it is best to try and start them in the wrong direction so that they will run in the correct direction. Even when running the wrong way they still draw in and exhaust through the correct ports.
In the case of the Detroit Diesel reported by rmw not only was it running backwards the intake and exhaust directions were reversed.

 

[patprimmer]

The Detroit is supercharged with a positive displacement blower that becomes a sucker when running backwards.

Regards
Pat
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[rmw]

I guess the blower on a Detroit Diesel doesn't care which way it is running, it will produce about the same flow in either direction.

On one occasion a relatively inexperienced driver had 'horsed' the clutch at a stop light and the engine lurched and I (sitting in a passenger seat) suspected that it had reversed and was running backwards. The driver put it in second gear - the normal take off gear, but, a gear position that was right next to reverse so it wasn't too uncommon for inexperienced drivers to put it into reverse thinking they had gone into second - and began to try to drive away. The bus moved backwards and I immediately directed him to put it into reverse and try it and sure enough it moved forward, much to his bewilderment. I had to get him out from under the wheel quicly and kill it (by putting it in 7th gear and bogging the clutch because the kill switch wasn't reliable).

I often wondered what it would have been like if we could have taken off backwards and worked our way up to 7th gear backwards

rmw

 

[TheBlacksmith]

In the 1960's most snowmowbiles had very little timing advance and weak starter ropes. More than one person managed to wind the starter rope backwards during a repair and the engines ran about as well in either direction. The centrifugal clutch would engage just fine and they would back into whatever was behind them.

 

[chief]

I used to ride A Czech bike called a Jawa back in the 60's. I was forever playing with the timing. On occasions IF i advanced it too far, it would run backwards. ,when i left out the clutch.
As too INLET VALVES in 2-stroke engines. I WORKED ON MANY MARINE DIESEL, SLOW SPEED DIESELS. I remember the British Polar Engine, built in Govan,Glasgow had a leaf type valve arrangement on the air inlet. These were naturally aspirated engines, about 200 rpm ,if I RECALL.

Offshore Engineering&Design

 

[SMOKEY44211]

Check thread 71-281562 You may be able to garnish some information it.----Phil