Question for the egg petrolheads... (engine musings content)

robin · Post by **robin** » Sun Apr 22, 2007 9:35 am

I had originally thought of using a cam pulley on the crank shaft, but this won't work with the standard cams because 1&4 and 2&3 are still 180 cam degrees out of phase, whereas your two stroke four cylinder engine will really be behaving like a two cylinder engine with 1&4 being one and 2&3 being the other, so they will obviously need identical cams.

In any case, the timing on even pathetic duration cams will be all wrong for two stroke I think - standard K has 244 crank degrees of duration which would translate into 122 crank degrees once you double the cam speed. Given that the exhaust is the same, that's 244 out of 360 in which the chamber isn't sealed. In the remaining 112 you need to compress and do the power stroke - maybe it would work?

If ever an engine was going to benefit from VVC mech, this was it!

Cheers,
Robin

Rich H · Post by **Rich H** » Sun Apr 22, 2007 10:22 am

Probably need to go for a smaller capacity, 1.4 perhaps. Also a new crank would smooth things out to allow the 4 to work independedntly instead of 2 pairs. New cams then on top of that and a vvc mech... As for petrol injectors, would a diesel be a better place to start prehaps? How much different is a rover diesel head to a K?!?

I have just found out that the porsche 924 block was originally a diesel...

robin · Post by **robin** » Sun Apr 22, 2007 10:29 am

Why smaller capacity? Also I wouldn't bother smoothing it out until the crank fails (it'll be taking twice the torque at any given RPM

).

Forget diesel.

Robin

hiscot · Post by **hiscot** » Sun Apr 22, 2007 10:33 am

there are quite a lot of differant engines some very clever but they require a major manufactor to take a chance and produce something differant and thats unlikey

http://conceptengine.tripod.com/concept ... index.html

thinfourth · Post by **thinfourth** » Sun Apr 22, 2007 10:35 am

Okay here we go into the lovely world of 2 stroke scavenging

Right the detroit diesels are on of the smaller marine 2 stokes the real big boys are a wee bit bigger

Walkin inlet manifold anyone?

Right robins idea of the 4 stroke engine being re timed to be a 2 stoke won't work worth a poo. As both your inlet and outlet are in the head so it short circuit the scavenging so you will end up with a load of crap at the bottom and loads of fresh air at the top. You really need some proper porting to clean out the burnt charge and unless you have port near the bottom of the cylinder.

Now as to rich's idea of two turbos too complicated What you really need to do is you set up an injector just before the inlet and time it so you stick in a shot of fuel just before the exhaust valve closes and you are still filling the cylinder so you mixing should not be too bad.

Now as to why a diesel is heavier basically it is because the firing and compression loadings are so much higher if you had the same loadings on your average petrol it would fall to bits

Rich H · Post by **Rich H** » Sun Apr 22, 2007 10:37 am

Smaller capacity as you will have twice as many power strokes and a smaller capacity would assist in speeding blowdown without making the inlet flow speeds too high. Big bore, small stroke I reckon would be the way forward.

Diesel internals should be stronger too.

Rich H · Post by **Rich H** » Sun Apr 22, 2007 10:42 am

I was thinking about last second injection after the exhaust port is closed off by the piston, charge blown in against the rising piston before the pressure defeats the blower. Direct injection would be a better option though.

Piston valving would be better for the exhaust but what about the temps on the edges of the port? I would imagine they would erode pretty quickly.

thinfourth · Post by **thinfourth** » Sun Apr 22, 2007 10:59 am

RICHARDHUMBLE wrote:Piston valving would be better for the exhaust but what about the temps on the edges of the port? I would imagine they would erode pretty quickly.

Assuming you go for a poppet valve in the head and a port in the liner then

Won't work as you need to open you exhaust before you open the inlet other wise your burnt charge goes out your inlet port which is bad.

You can't change your timing with ports with regards before and after BDC as your timing is controlled by the piston Where as with your cam you can easily change your timing before and after BDC.

robin · Post by **robin** » Sun Apr 22, 2007 11:05 am

The way modern heads works with overlapped cams means that they have the very same problem (of charge blowing out the good rather than the exhaust). The way the charge is angled means that the incoming charge is forced to the bottom and the exhaust is pushed up the cylinder. It helps to have already opened the exhaust valve for a while before you start, though, so that the chamber pressure has dropped to something less than the (super charged) intake pressure and so that thre is already a stream of moving in the exhaust manifold.

Cheers,
Robin

Rich H · Post by **Rich H** » Sun Apr 22, 2007 11:51 am

thinfourth wrote:
RICHARDHUMBLE wrote:Piston valving would be better for the exhaust but what about the temps on the edges of the port? I would imagine they would erode pretty quickly.
Assuming you go for a poppet valve in the head and a port in the liner then

Won't work as you need to open you exhaust before you open the inlet other wise your burnt charge goes out your inlet port which is bad.

You can't change your timing with ports with regards before and after BDC as your timing is controlled by the piston Where as with your cam you can easily change your timing before and after BDC.

Can't change the exhaust port timing but you can change the inlet timing. exhaust will be open either side of BDC and the inlet could open say at BDC as the cylinder pressure dissapates to blow the old charge out. Piston starts to rise exhaust port is closed inlet still open against the blower, inject fuel (Big injectors required!)