/// Atkinson cycle diesel tdi ? ///

Hi all

Other than the Toyota Prius having a hybrid drive with a motor one of the big reasons its engine is so fuel efficient is that it runs off the Atkinson cycle rather than the typical Otto cycle.

I was thinking of having a custom camshaft ground to basically turn our little TDI engine into an atkinson-cycle diesel ... something like that about to get crazy good fuel efficiency without sacrificing too much power because diesel typically have good torque to start with so maybe missing some won't be that much of a problem unless of course you're trying to Hot Rod around.

I tried Googling Atkinson cycle diesel but I have not found anything which means maybe my Googling is not that good or maybe it's never been done?

I actually contacted a custom camshaft manufacturer earlier today with my crazy idea because basically all it takes is a custom-made camshaft.

However before getting into all the expenses surrounding that what do you all think about that idea the camshaft to effectively make an atkinson-cycle diesel engine?

If I get this put together and made I would name my Prius Benedict Arnold Prius because it would be able to roll coal which is actually totally against what most Priuses are about I think it would be hilarious and also kind of fun to get super economy and performance what do you think?

Thanks in advance for any and all help on the Atkinson cycle as it relates to diesel

Andrew

Izat where the intake stays open for part of the compression stroke?

The Prius is more of a mimic of the real thing. And the diesel process is already much more efficient. The emission controls strangle it though.

KLXD said:

Izat where the intake stays open for part of the compression stroke?

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Yes, I think the intake stays open for about 30% of the pistons upward compression travel.

It's hard for me to wrap my head round why that helps but in a gas engine appears it does .... something about compression ratio VS expansion ratio (acts like a small engine for compression and fueling purposes BUT has the full length for the power stroke ....).

I could never really find a comparison between engines identical except for one running the atkinson cycle vs the otto cycle.

By chance do you or anybody here know what a TDI camshaft is made of ? .... specifically is it cast or machined?

I spoke with a custom camshaft co and they needed the technical specs of the TDI cam in order to possibly make one with an delayed intake valve closing (thus making the engine effectively work more along the atkinson cycle ...

The other question I have tho is .... do the pistons have to be changed to up the compression ratio since you are effectively letting about 30% of the air out before compression starts (then of course that my throw the engine out of balance ( if you increase compression by making pistons "thicker" of course maybe shaving the head may do that easier provided piston and valve clearance and is OK ).... I guess 1 step at a time ...

TDImeister could weigh in on why this is not readily applicable to the modern diesel engine the way you think it is.

I really doubt you'd see more than a couple percent if that, and even if you got say, 10 percent better FE, you're spending a couple thousand minimum to get it running that way. How many hundred thousand miles does it take to break even?

To clarify my above statement about the Prius (and others) not being a "true" Atkinson cycle engine, but a mimic of one:

The original idea involved some pretty complex crankshaft and extra pieces to make the effective "strokes" of the piston differ depending on which cycle the engine was currently on. Modern engines do it differently, as they do it with clever variable timing of the valve lift. There are other twists on it, most notably the Miller cycle, and now we have the Budack cycle. The key difference here is that the Atkinson has no forced induction, the Miller and Budack do. So, the Miller for sure can and has been made in diesel versions (big ship engines, mainly), I am not sure the Budack has been applied to diesels but given it is from the Germans it is highly likely that it has been.

The Miller/Budack engines have some differences with regards to valve lift and timing, enough so that there are different patents, thus the different names, but are substantially the same basic premise as the Atkinson: lower the amount of "work" involved in the compression stroke to make the engine more efficient. But where the Atkinson just means a weaker engine (less torque) and thus works well paired with an electric (high torque) motor, the Miller/Budack make up for this loss of power with either a supercharger or a turbocharger.

Diesels, simply by design of not having much margins of space between the piston and valve, do not have as much wiggle room to work with. So any variable valving (which some of the later TDIs DO have), won't be able to as easily make as much of a change. Plus, lowering the compression stroke too much means you'll need even more forced air scavenging to make up for the loss, otherwise combustion simply will not happen.

The ability is already present right now... actually probably a decade ago... to boost the fuel efficiency of a diesel beyond what it already is. We could all be driving ALH-like powered cars that tag 70 MPG quite easily. Without any fancy vario-compression-valving-etc. wizardry. But the emissions compliance would never fly. The current regulations FORCE them to burn much more fuel than they actually need to move the car down the road, in order to lower NOx and keep the DPF working.

Andyinchville1 said:

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By chance do you or anybody here know what a TDI camshaft is made of ? .... specifically is it cast or machined?

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It is machined after being cast. Just like most other cams. The only ones that aren't done this way are the type that have lobes pressed on a shaft (mass production, something indy shops can't duplicate) and billet, which are cast as a block then machined and won't really have metallurgic differences from the original cast then machined, just much less waste during the machining. (If you're thinking that I think the 'billet' label is a bunch of marketing BS, then you'd be correct.)

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The other question I have tho is .... do the pistons have to be changed to up the compression ratio ...

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Yes. The earlier comment about shaving the head for compression ratio is that no, you don't. If you ever look at one of our TDI heads, you will see that it is already flat. The only machining done to a TDI head surface is to ensure it is still flat - that is, to correct any warping that has happened.

The combustion chamber is completely contained in a well in the piston top. The injector spray pattern sprays into this. If you shrink it to increase the combustion ratio, you run the risk of the fuel spray impinging on the piston surface. It's an aluminum piston, so you run the additional risk of melting the piston. This would be an extremely tricky thing, so better make sure you have it all perfect or you'll have an engine that grenades itself in just a few hundred or thousand miles.

As has been mentioned above, the diesel is already more efficient than even an Atkinson cycle engine. Not to mention the additional power available in a good running diesel.

Also to be considered: gassers don't normally run in a super-lean condition like diesels (try 100:1 at idle) because they can't ignite the fuel in that regime. And the turbocharger will allow this lean burn condition even at high power. At least, with our smoke map, combustion remains lean, efficient, and better than most any gasser can do.

The Atkinson cycle is great for a gasser because it allows more expansion. The diesel is already there with more expansion due to the compression ratio. Increasing that ratio really won't get you much more, just more NOx emissions, and we all know how much the EPA loves that stuff.

Nice discussion, and good ideas. It is difficult to see where you will be able to come up with a significant gain without spending zillions to get there, like the big car companies can do. Sometimes the possible reward just isn't worth the huge effort to get there, at least not for an individual.

Mazda's Skyactive diesel is an interesting concept at which they started from a lower compression ratio at the beginning start up position. Check into that concept and see what they're doing. It is very interesting as well. But also note that they can't get it to pass current emissions hurdles, or they probably would have brought it here as they originally intended. Well, they intended until VW pooped their britches and nearly destroyed the passenger car diesel market for everyone in the USofA. So sad....

Cheers!

PH

Nothing found because it must not work!

Not a single reference to the ridiculous explanation that the intake valve remaining open for part of the compression stroke makes some of the original intake charge available to the next cylinder.

I'm so proud.

Really, no sarcasm.