Cam regrind... for efficiency?

[Turbinepowered]

Okay, so, I'm toying with the idea of having a cam made up to make a Miller cycle 1.6TD. Basically, making the cam such that the effective compression ratio of the engine is down there in the 18.5:1 or so range, but leaving the head, head gasket, and swirl chamber volume all stock.

I figure this way I can run the higher boost pressures without blowing out the headgasket, but at the same time I get the full 22.5:1 expansion ratio, which should increase thermal efficiency.

Crazy, cool, stupid? Whatcha guys think?

[burn_your_money]

What's a Miller cycle?

[Turbinepowered]

Miller cycle is a modification from the Atkinson cycle, which was developed as a four stroke cycle that had a low compression ratio but a high expansion ratio.

Atkinson cycle accomplishes this by holding the intake valve open during the first part of the compression stroke to effectively reduce swept volume, reducing compression. Miller cycle does the same thing, but uses either a super or turbocharger to make up for the lost air(and fuel) volume. A tall expansion ratio allows for better utilization of the heat-expansion of the spent air-fuel mixture, and a low comression ratio gives lower peak pressures and less chance of detonation in a spark ignition engine (Lower peak pressures for us).

Doing something like this should lower EGTs and increase the thermal and fuel efficiency of the engine, with a small hit to low-end power before the turbo spools up. Coupled with a VNT controlled properly, you probably wouldn't even notice that bit of low end lag. I think.

[lyeinyoureye]

I bet you'd see a near TDI BSFC map with a cam like that and a properly setup variable vane turbo. The only problem I can think of would be harder starting in the cold. Would the valve staying open that long imply more risk in terms of interference?

[rabbid79]

Very cool.  Didn't Mitsubishi use it in one of their cars in the mid 90's?

[lyeinyoureye]

Yep, and they're bringing it back now.

[Turbinepowered]

I bet you'd see a near TDI BSFC map with a cam like that and a properly setup variable vane turbo. The only problem I can think of would be harder starting in the cold. Would the valve staying open that long imply more risk in terms of interference?

I don't think it'll cause interference; wouldn't it just be staying open an extra couple of degrees crankshaft?

Anywho, I was planning on combating the cold starting issue by adding an intake preheater screen, like come on come Cummins direct injection industrial and heavy duty engines. Basically it would preheat the air, then the air would further be helped by the glow plugs. Might need to cycle them a couple times, but it should fire up.

[Turbinepowered]

Perhaps to add a bit more clarity, this engine would be effectively a dual cycle Atkinson/Miller engine. At low RPMs and before the turbo spools, the engine would be running on the Atkinson cycle. Once the turbo begins to produce boost, the effect is the same as a Miller cycle engine, minus the power-robbing belt-driven supercharger.

Might also make for an excellent candidate for a dual turbo system, pairing an ultra-small turbocharger (K03 or smaller, I'm thinking) with something more in the K24 range, or maybe something like a VNT-XX on the big end. Because of the decrease in effective swept volume on the intake while running without boost, the turbocharger I expect will be harder to spool up. Running duals would allow me to size a super-small version and add in a larger turbo to give it's grunt later on.

[ezekiel]

interesting...

[burn_your_money]

Thanks for the explaination.
If you have a healthy engine I don't think you'll have a problem with cold starts 90% of the time.

I think you should go for it, I'd be interested in one if it works although maybe less duration since I'd be running a K14

[Turbinepowered]

Seems the consensus is to try it, so now I just need to see who I can get to grind up this lovely little gem, and for how much.

I guess it's time to contact some of our excellent vendors, particularly the ones that advertise custom performance cams.


I'm planning on putting this into a fully rebuilt engine, honed with probably some total seal rings, fresh valves, new nozzles in the injectors, and obviously things like a new headgasket and headbolts. Might go with studs, not sure yet.

[burn_your_money]

Right on. Please keep us posted

[Darth Garry]

Sounds like a neat project to me.

My only question would be why not try putting it into a currently running engine first?  If you have a motor that you know is going to run a certain way, you will be albe to change the cam and measure the difference.  

If you put it in a freshly rebuilt engine, it may be more difficult to gauge the difference that this cam will make.

I'm real interested to hear how it turns out though, either way.

Garry

[Turbinepowered]

Sounds like a neat project to me.

My only question would be why not try putting it into a currently running engine first?  If you have a motor that you know is going to run a certain way, you will be albe to change the cam and measure the difference.  

If you put it in a freshly rebuilt engine, it may be more difficult to gauge the difference that this cam will make.

Oh I'm keeping the stock TD cam as well, for comparison; the engine needs a rebuild, both because it was in a front end collision and because it had some blowby issues on a couple cylinders. Might as well do the whole thing while it's not in a car, you know?

I figure I'll get the Miller/Atkinson cam ground from a fresh blank, so I can swap back and forth between stock and modified. Probably going to go looking for a dyno in my area, to get it down on paper.

[ezekiel]

odd that the Mazda2 "Miller-Cycle" engine has no supercharger, making it an Atkinson-Cycle engine, but they're still calling it a Miller-Cycle.

and that the Ford/Mazda and Toyota Hybrids use an Atkinson-Cycle gas engine in their hybrid drivetrains.

knowing that, kinda makes you think the hybrids should be getting even better mileage than they do.