Scottish Elises

Been bored again today, you all know that is bad, the devil makes work for idle hands etc...

The thought for today was 2 stroke engines. Big diesels are 2 strokers with a big supercharger providing the initial compression into the crank case, then the piston opens the transfer port near BDC and the cylinder is blown down by this fresh charge and out the zorst valves. The fuel is injected and you have combustion. Power stroke every crank rotation.

With me so far?

Now we all know diesel is good for 3 things, lighting bonfires, big trucks and winning le-mans, so why can't you do the same with petrol?

OK the fuel will need to be injected into the inlet just before the transfer port and it will have to be injected quickly (Multi point injectors to get atomization?) while the port was open to stop fuel getting back down into the crankcase. (Crankcase fire anyone?!?) Then traditional sparky plug in the top. You then get a supercharged 2 stroker fuel injected petrol. Twice the potential power for a small weight penalty?

Problems:
Lubrication: (There is always time for lubrication! /evolutiiion) how do you lubricate the bottom end without burning loads of oil?
Piston valving is always a weak point: dunno about advances but the RX-8 wanky-rotary engine has similar and thats quite good now.
Timing of transfer is basically set, no fine tuning of inlet phase.

Advantages:
Power stroke every rotation per piston: Potential for twice the power. OK I know this isn't exactly true but more power or less weight.
No inlet valves, one camshaft, lighter simpler valve train, less to go wrong.

Are these the ramblings of a talented, under employed engine genius or utter crap?

Answers on a postcard....

Like this...sort of...but cut in half and with an exhaust valve.. IYSWIM


Found on Wikipedia. This design was scrapped due to high fuel consumption but with the advent of EFI...

2 strokes not so hot on the petrols but the diesels i could bore you to tears.

first thing first all 2 strokes are supercharged in one way or another or it won't work. Your big diesels use turbos your chainsaw uses under piston compression and needs reed valves.

Now on the big diesels and i mean big not the little ones you find in buses i mean proper big. You have a diaphragm plate between the cylinder and the crankcase which means you don't have to pressure seal your crankcase and you get no cross contamination. But this is only practical with a crosshead engine but on the big diesels you need the crosshead anyway as you could not get the bore/stroke ratio without battering the con rod off the bottom of the liners. 960mm bore 2500mm stroke unless a superlong stoke which are a faintly ridiculous about 3mtr stroke.

Now for your wee screamer engine you can either have oil in the fuel as in traditional 2 stokes but it means very very pap emissions or you can have very long piston skirts to keep your scavenge ports closed and have a normal lubrication system. However the long skirt would mean you need a long stoke so not so good for revs.

Now all two strokes scavenge out the exhaust gas with the fresh charge coming in and you always get a wee bit of fresh charge going out the exhaust. No probs in a diesel as you have no fuel until you have full compression but it is a problem as the fuel comes in with the new charge so some buggers off out the exhaust.

So what you end up with is a engine that is hugely inefficient and smokes like a barsteward but if you don't care about emissions or fuel economy then they be great.

Do note 2 strokes are banned in F1 and the moto GP bikes when they were 2 strokes were 500cc and they went to 1000cc 4strokes with a small power drop. Should tell you something.

Still waiting for a nutter to drop a snowmobile engine into a kitcar or a elise

Hmm, how much power does a snowmobile make then?

http://www.aaenperformance.com/V4_racing_engine.asp

about 200bhp but look at the weight 44kg

V-4 1025cc 275bhp @ 10,000

That makes the torque at 10k rpm around 140 ftlb witch is stillquite respectable. Bit of a pain in traffic though...

Another point about 2 stroke petrol engines (in 500GP bike form anyway) was the lightswitch power delivery. The 500s were much more difficult to ride, despite having 190-200bhp, compared to the 230bhp of the first Honda RC211V 990cc 4 stroke MotoGP bike (this was even before they nailed TC on them)

Towards the end, the 500 boys were into all sorts of wheezes like injecting water into the expansion chambers of the zorsts to fool the engine into thinking they were larger at lower RPM, giving less of a whack when they came on song.

They were legendary, widowmaking, evil, magnificent things and a real black art to engineer (compared to the more F1-esque 4 strokes), but they had been a total anachronism for at least 10 years when they were finally killed off - no manufacturer had made a road going 2 stroke over 250cc since the 80s.

Lots o power, lots o smoke, almost uncontrollable Great for motorcycle racing loonbags, but sucidal in a car

Shug wrote:Lots o power, lots o smoke, almost uncontrollable Great for motorcycle racing loonbags, but sucidal in a car

When you fitting one then

On a recent boat trip I was really bored and read a book about marine diesels which was actually very good . The 2 stroke diesel you speak of are Detroit diesels which though powerful are noisey ( restriction in exhaust can make the engine inefficient ) and do not last as long as 4 stroke diesels.

If I remember correctly the book basically explained that the problem with petrol is burns very explosively , the good old diesel burns much slower ( I think 3 times slower ) and controlled giving a longer push on the power stroke hence why diesels are so torquey ( a fact I never new).

It stated the temperature in the combustion chamber of a diesel was approx 800 degrees ( for example ) to use petrol in the same engine it would have to be lowered to 500 degrees to avoid detonation. So you would have to lower compression making the engine less efficient. To get the power back you would have to raise the RPM.

Cooling and pressure was an issue in the diesel , hence heavier for a given engine size but I cannot remember the explanation.

Makes sense in my head.

Cheers all. Knew it was crackers.

Next problem, what to drop into a 924, but that is a different problem....

Right the problem is blow down on the cylinder. You loose some charge through blowing the old charge out, so why not have a supercharger do the blowdown then a new charge introduced rich through an inlet valve? (Even better why not suck the charge out? Too hot though, sh!t idea.)

The blower charge is all thats lost and then once the exhaust is masked the inlet charge in squirted in also by blower (Or it'll get blown out of the cylinder!) So the charges are pushed in each time. Might need 2 blowers though... Or a turbo as well...

Hmm. Not sure this will work 4 stroke is just so easy... but all that wasted movement...

I know there is a cycle that uses a blower and extends the inlet cycle into the compression phase against the pressue of the blower. Can't quite remember what it's called. Still 4 stroke though.

Need to think more...

RICHARDHUMBLE wrote:Right the problem is blow down on the cylinder. You loose some charge through blowing the old charge out, so why not have a supercharger do the blowdown then a new charge introduced rich through an inlet valve? (Even better why not suck the charge out? Too hot though, sh!t idea.)

The blower charge is all thats lost and then once the exhaust is masked the inlet charge in squirted in also by blower (Or it'll get blown out of the cylinder!) So the charges are pushed in each time. Might need 2 blowers though... Or a turbo as well...

Hmm. Not sure this will work 4 stroke is just so easy... but all that wasted movement...

I know there is a cycle that uses a blower and extends the inlet cycle into the compression phase against the pressue of the blower. Can't quite remember what it's called. Still 4 stroke though.

Need to think more...

Maybe less thinking might be better

Nah. I'm good at thinking, but the results are always tosh!

Wine is the way forward tonight!

Take an FSI or GDI engine.

Get some new cams made so that valves open twice per revolution not once (I am sure this could be made from normal cam blanks) with suitable inlet and exhaust durations to suit your mad scheme.

Fit a super charger to blow air (not fuel) into the cylinders.

Maybe use reed valves in the intake so that when cylinder pressure exceeds inlet pressure the valve shuts.

Then use a megasquirt to run the fuelling, job done.

Have you done it yet?

Cheers,
Robin

Basically negating the blowdown emissions problem! Fantastic! Rather than mince about with expensive cams, how about making a cam pulley fit the crank? jobs a carrot. Pair of verniers on the top for tweaking.

Reed valve in the inlet tract of each port, let the valve still do it's job for the high pressure bit (12.5:1 comp ratio!) but the reed valve can deal with the initial pressure rise.

Might have to be inventive on the timing. So we need the exhaust open just before BDC then the inlet open just after, the blower pushing the old charge out and the new air in as the piston start to rise. Exhaust closes, blower still pushing air in, inject fuel, bang, etc.

Hmmm, where to find a GDI... and a machine shop... and some time... and some money...

Bollocks