Cam regrinds for Atkinson cycle? - Fuelly Forums

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Old 07-08-2007, 09:47 PM   #1
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Cam regrinds for Atkinson cycle?

I stole this from vwdiesel.net and was wondering if anyone here had thought about it. On SI, it'd much easier to implement than diesels, since starting doesn't depend so much on CR, and having a longer expansion ratio than effective compression ratio allows for better thermal efficiency. The torque curve, and power, would drop proportionally to the decrease in effective CR, but I think it may be worthwhile. Kinda like how gearing is good for an effective increase in mileage at the cost of acceleration/power in that gear, this would be good for mileage across the board at the cost of peak torque across the board. Nice for someone who does a lot of city driving.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 07-09-2007, 07:27 AM   #2
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The goal is to have a greater expansion ratio than compresion ratio, right? So you would regrind the cam to close the intake valve later, right? If you did that, you would also need to do something to increase the compression ratio, I think. Shave the heads or the deck or both... Anyway, cool idea.
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Old 07-09-2007, 07:36 AM   #3
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I'd love to do an Atkinson conversion on a small block Chevy V-8 because so many parts are available. Use high (> 13:1) compression pistons and heads, smallish valves for high swirl and an Atkinson cam.

I'll bet that if you took a GM 4.8 and geared such it right, you could get 30 mpg highway in a GM full size pickup.........
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Old 07-09-2007, 07:44 AM   #4
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No one is thinking of variable valve timing - be cool to add the cam adjusting sprocket from a Toyota VVTi engine to a regular engine and then take control of the timing. I wish I could figure out when it is doing what when it adjusts - maybe we need to get into programming the Scangauge with the special codes so we can monitor other engine ECU functions.
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Old 07-09-2007, 07:45 AM   #5
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omg, the discussion from vwdiesel.net you cited would just be a continuation of camshaft/valve timing juggling that has existed for a long, long time. No need to relate it to Atkinson cycle at all.

At Toyota, evidently either public relations people or engineers who wanted to put a name on an engine design attached the words 'Atkinson Cycle' to an IC engine that had an expansion ratio greater than the compression ratio and ignored the fact that the Atkinson design accomplishes four cycles with only one crankshaft revolution versus the two crankshaft revolutions required of the Otto Cycle design. Without this feature, Toyota Prius engines remain Otto Cycle engines. One of the chief objectives and benefits of the Atkinson Engine design is reduced internal friction enabled by their design. Toyota is just throwing around a term implying use of Atkinson engine design while only incorporating a couple of features that are remotely similar in both engine designs.

Both features I am referring to have been used for ages in Otto design engines. Those features are slightly offset crankshafts and use of valve/camshaft timing to vary the duration of compression and expansion cycles.

Offset crankshafts have been used in many engines since the very early 1900s, especially inline engines. V engines accomplish the same effect with offset piston pins. Both these methods succeed in reducing loads on the cylinder walls somewhat.

Valve timing has always been a very important area in Otto engines to tailor torque curves to intended use. That is why performance camshafts exhibit characteristics very undesirable in FE oriented use. Proper valve timing can and will produce an effective expansion ratio greater than the effective compression ratio in an Otto engine.

That being said, if one can design a camshaft with optimum valve timing and valve lift suited perfectly to their engine, intended purpose, and driving style, I believe there is considerable room for improvement in Otto engines. Many Otto engines incorporate VVT(variable valve timing), including my own.

I am pursuing this avenue for Old Reliable at this time. The present camshaft timing in the car is slanted considerably toward performance and higher rpm capabilities than with FE in mind. I will try and use my years of experience to help design a camshaft more suited to my purposes.

I am sure the camshaft will complement the ultra-high trans gearset that I have ready to use. Factory gearing requires 2400 rpm at 56 mph. I will be operating my engine 1850 rpm at 56 mph with my new gearset. This requires shifting the torque range to a considerably lower rpm. That will be the job of my new camshaft.

My engine will not be an Atkinson engine, just as Toyotas and some others are not Atkinson engines.
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Old 07-09-2007, 07:47 AM   #6
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Looking into this a few weeks ago I came to the conclusion that to do an effective atkinson conversion would require an increase in effective compression ratio with high CR pistons. The effective compression ratio begins after the intake valve closes. Since atkinson cycle closes the intake valve so late there is like 60% of the normal otto cycle compression available to the fuel mixture charge, to gain that back a proportionately smaller combustion chamber is needed.

Just changing the cams would, I think, get some benefit but the significant decrease in effective compression would result in lost potential. OEM atkinsons have like 13:1 ratios but I do not believe these to be effective ratios.

I read on eng-tips.org that someone offered atkinson kits for small block chevys back in the 70s but were frought with all sorts of problems, mostly related to carburetors not liking the intake pulse disruption. With todays high ouput FI engines, seems an atkinson conversion could work really well.

Output is generally 30% less than with otto cycle for the same size engine, so the engine would have to be sized appropriately. An existing overbore and/or long stroke with high compression piston kit with atkinson regrind cams should be a good starting point. Also, atkinson cycle engines typically have lower rpm operating characteristics.
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Old 07-09-2007, 01:08 PM   #7
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@Bill- Well, I think it depends. I've noticed that a lot of engine families started out with high CR designs, and as FI/etc was introduced the manufacturer dropped the CR. So, provided the difference was such that it went from an effective CR of 11:1 to 9:1, I think it'd workable.

@Sludgy- Yep, gearing should allow that np. As long as the driver doesn't mind a bit more shifting.

@Jan- That'd be sweet, but I have no clue what modifying/swapping a non-vv head for a vv head would entail. For some engine families it's probably impossible, and for others, may require who knows what. It's definitely something to research.

@CO- Tomato, Tomatoe.
Everything gets flipped around, that's the evolution of jargon. The Prius isn't a hybrid in the sense that it uses two different fuel sources, so they slap hybrid electric vehicle and make another category. Just like they did with the four stroke Atkinson cycle. But yes, for all intents and purposes, it'd just be changing the cam such that the intake valves stay open long enough to allow some air to flow out of the cylinders. Reducing the effective displacement and CR of the engine while having a expansion stroke that was longer than just about any other engine of similar effective displacement.

@beatr- Like I said to Bill, I think if an engine family went from some higher CR at one point to a lower CR later on it would doable. Although high compression pistons would likely be a great addition, have a near stock effective CR w/ a longer expansion stroke.
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Old 07-09-2007, 03:38 PM   #8
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Along with shortening the effective compression stroke I will have a longer effective expansion stroke with my new camshaft. All modern gasoline engines open the exhaust valve well before the power stroke is complete. I will delay that opening to extend the power stroke. In an engine that is not being punished and operated at relatively low rpm, it is unnecessary to use a considerable portion of the power stroke pushing exhaust to the extent that is common. There are many compromises in factory engines to allow the various uses and driving habits required by the general population, therefore room to wiggle if building for a purpose.

Make any sense?
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Old 07-10-2007, 06:52 AM   #9
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All modern gasoline engines open the exhaust valve well before the power stroke is complete.
Really? Wow, I did not know that. Seems pretty wasteful.

Is VVT on the exhaust side usually made to keep the exhaust valve closed a little longer at low loads?
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Old 07-10-2007, 12:02 PM   #10
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The previous VVTi engines (Variable Valve Timing intake) only had intake valve adjustments but the new bigger Tundra V8 engine now has intake and exhaust variable valve timing. Yes usually the exhaust valve opens pretty quickly - while the piston is still traveling downwards so to get as much of the pressure out of the cylinder as soon as possible to enable it to operate at higher RPM. At low RPM a later opening would be a great idea but without VVT it would probably not have very high rpm capability. Getting the fuel to burn faster would also help so that the exhaust valves are not getting flamed when they let the still burning exhaust gasses out. If you wanted to wait even longer and have the burning gasses cool and have them suck the piston back upwards on the exhaust stroke that would be interesting too.
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