4 into 2 stroke using cam....would it run? 1

[obanion]

Here's a idea. Take a typical gasoline 4-stroke motor. Make a custom cam, that is also made and installed such that it turns at the same speed as the crank (2x normal). The cam timing would be something like this:

110ATDC, exhaust valve opens
140ATDC, intake valve opens
140BTDC, exhaust valve closes
110BTDC, intake valve closes

Obviously, ignition events will need to be doubled as well. If you have a waste fire ignition, no need.

Only one thing missing, a means to push air into the cylinder during the overlap period. Supercharger or blower.

Cam timing would need to be adjusted of course, to minimize excess blow through of air and especially fuel (which will be port injected).

Additional thought, use a variable cam timing system. Can retard intake cam up to 40 degrees. You could modulate power this way (no throttle). Retard the intake cam, and the intake valve closes later in the stroke, letting the cylinder blow more air/fuel back out the intake valve, lessing the total charge in the cylinder, serving just like a traditional throttle.

Concerns are max power from a given displacement. Efficiency isn't much of a concern (race motor).

 

[wurlyvalve]

Hi,

Sorry, very tired, so my answer will be less enlightening than usual....

I know a lot of drag racing motorbike boys that posted on macdizzy 's forum were well into taking 4 stroke motorbike engines and changing the cam drive to 1:1. There was then two methods of putting the intake charge into the top end, either use all four valves (per cylinder) via a compressed plenum (supercharger etc) and make custom injection system and bore an exhaust port into each cylinder and make some expansion chambers. Or use just use the two standard intake ports and use a fuel injected engine (so they could use the original intake system) and pump the exhaust gasses as normal through the exhaust system.

The figures they were giving (and this was 5 years ago and highly un-developed) were similar to 4 stroke turbo figures, about 400-450hp. But the big difference was the 2 stroke conversion was peaking at about 6-6500rpm. Dare say they've sorted this out a bit now. Might be worth checking macdizzy to see?

Cheers,
Glen.

 

[jomor]

I would be interested to see what you come up with, I have a Detroit Deisel 8V71 that is similar to what your considering. Two Stroke Deisel with cam/blower etc. the 92 Series (92 in^3/cyl) also have a turbo that feeds a blower.
Hope this might help with your research.

jomor

 

[obanion]

wurlyvalve, thanks for the reference. I'll see what I can find out.

I'd think the approach using a exhaust port would have a much better volumetric efficiency, having much more flow area, and doing a top to bottom filling. However, it would require a completely custom engine build.

The second approach, pretty much sounds like what I suggested. That could be tested on a cheap junkyard motor, only needing a pair of custom cams (DOHC motor) and a change to the cam belt pulley on the crank to have 2x as many teeth.

 

[wurlyvalve]

Hi,

I think they were building a collosal plenum above the engine (where the fuel tank would sit) and pressurising it via a turbo(s) off the end of the expansion chambers. They then connected all four valves to the plenum. Bored a hole in the block/barrel and tigged in an exhaust header pipe. They kept the std wet 4 stroke bottom end and just pumped the top end with the high pressure plenum thus doing away with the nasty task of welding in a load of transfers and having a smilchy exhaust.
They were reasoning that the 4 strok bottom end is bullet proof in a 2stroke environment as the pistons seldom experience a low pressure stroke, hence giving the rods an easier time (potentially giving them a far higher redline).
Alas, I never followed the progress through. I switched passions and stopped following the 2stroke forums with any detail. Since then Macdizzy has decided to charge people to use the site.
Another fav was taking ski mobile engines and working on them..... ?

 

[DesignOutsourcing]

It's a lot easier to put extra lobes on the cam(s) than to change the cam drive. Modern 4-stroke racing engines use double-lobed, crank-speed cams to reduce vlave train drag. good luck!

 

[GregLocock]

Really? What value is a double lobed cam running at crank speed when your valve events need to operate at half crank speed?

Cheers

Greg Locock

 

[DesignOutsourcing]

Oops! Sorry-- late night brain fart! Obviously it's a quarter-speed cam instead of half-speed if double lobed. There is less drag at lower speeds.

 

[crysta1c1ear]

I read about one of the car manufacturers working on a 4 stroke engine which switched to 2 stroke operation when extra power was required, (eg overtaking,) its main advantage being that it allowed the engine to be downsized and hence there were associated efficiency (=fuel economy) gains.

 

[TTmotors]

I wonder if there are any possibilities of quarter turn four stroke engines...

 

[crysta1c1ear]

What do you mean by ¼ turn 4 stroke engines?

There are 4 stroke engines which which complete all 4 cycles in one revolution of the crankshaft if that is what you mean. The advantage is that RPM is halved.

Imagine playing footsie under a table with a secretarty at lunch.

The ball of your foot can fit in her instep while the ball of her foot fits in your instep: total volume of the space between is zero. This corresponds to complete evacuation of the exhaust gases.

As you push your feet forwards, air is drawn into the gap between the feet. The ball of each foot meets the heel of the other. This is maximum volume.

As you push your foot further forwards and swing it round to her side say, the heel of you foot in now in her instep, and the heel of her foot is now in your instep. The volume between the two insteps is now small, but the heels are a worse fit than the balls of your feet. Hence the air is compressed, but unlike the exhaust case, this time the volume is non-zero. It gives your compression ratio.

As your foot carries on round, and starts to come backwards relative to hers the gap between the feet start to grow again. Your heel is slipping from the instep back to the ball of her foot and her heel is slipping back to the ballof your foot, ie maximum volume gain when you are both ball to heel of each other's foot.

Finally, as your foot pulls further back, you will end up in the initial position, the balls of your feet in each-others instep, exhaust gases expelled.

As you swing you foot round in one circle, you go through all four cycles, from zero volume (complete exhaust evacuation) when the balls of the feet sit in each other insteps, to compression when heels sit in the insteps 180° later and back again.

If you actually do it - rub feet with somebody under a table as described - you'll find the feet swing from side to side as they go round as well as just moving backwards and forwards, ball to heel.

What this means is that the feet are say more towards yours during the intake phase and more towards hers at the exhaust stage. The effect is to provide a cold intake area remote from the hot exhaust area. This helps to prevent preignition.

The side to side part of the feet's motion also means that the engine needs no valves. As the feet move towards yours, your foot can reveal the cold inlet port. As they move towards hers, her foot reveals the hot exhaust port.

End result: a 4 stroke engine which fires every revolution, needs no valves, has a cold inlet, completely evacuates exhaust gases, etc.

What does etc mean?
Self supercharging. Motion is rotary rather than linear. Just forgot another important one, and have to go to eat!

 

[TTmotors]

That is such an odd description of a rotary.... heh

I was more talking about the posibilities of quarter speed double lobed cams.. or mayby quad lobe 1/8th speed. What are the fricitonal losses of cam bearings? Has anyone tested it? It seems a design like this would be limited by packaging only and the benifits would be very great in limiting momentum of the cams. Throttle response clutch wear and acceleration as well as decceleration would all be improved. Especially with lightened cams to further reduce rotating inertia. This seems like a very good solution to the innefficient drive trains.. Does anyone see a problem with it? Perhaps larger base circles might need to be utilized?

 

[DesignOutsourcing]

Quarter speed, double lobed cams are already being used. I heard on this or another forum that some F1 teams are using this setup since the frictional losses start becoming very significant at the 17-19,000rpm that they use. Can anyone confirm this??

My initial observation was that it is easier to add lobes for a 2-stroke conversion than to change the cam drive. cheers!

 

[SMOKEY44211]

Back in the late 50's early 60's there was a guy named Schaller that marketed a 1/4 speed cam for small block Chevs and H-D motorcycle engines. Claim was that it would cycle faster for a given valve spring pressure vs 1/2 speed cams. I think Mickey Thompson used one in his Chevy powered Indy car in 64. In the end it didn't seem to offer any advantage over 1/2 speed cam.-------------Phil

 

[Techno97]

I question the usefulness of this. Your keeping the worst part of a 4-cycle, the valve train. You may get more power per liter but you'll have to cram that power past those valves. Not to dissuade you since I'm following a similar course.

 

[dunno]

We do seem to have defaulted back to four stroke engines!
I thought the subject was on converting a four stroke to two stroke - ??

Now if you use a two lobe cam at quarter crank speed, that's the same as the original four stroke single lobe at half crank speed, - right?

However, a two lobed cam at half crank speed will be right for two stroke operation, ( opening the valve once per revolution of the crank).

This is therfore the same as a single lobe cam revolving at crankshaft speed!

I'll say no more, I don't want to get confused as well!!

 

[icrman]

It may work but not very high VE. Thats why the blower on the detroit diesel, and it is uniflow design for scavenging. Blow down from the top while trying to scavenge to the top at the same time, is questionable.