Split cycle HCCI engine idea

  • Thread starter awdgsx
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  • #1
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Just wondering if an idea I've had for a few years would actually work.

If I had an engine where compression took place externally and the engine was only used for combustion for example. This engine would be direct injected. The fuel would be injected a very high pressures (common rail) just before T.D.C. into a combustion chamber that was at atmospheric pressure for atomization of the fuel. The compression charge would then be introduced at very high pressure from the center of the combustion chamber for instant compression ATDC igniting the premixed fuel-air charge.

This engine would have complete control of the amount of compression per cycle, the amount and timing of fuel injection, and slight control of the temperature of the compression charge.

My thoughts are that as a diesel engine, it could be operated in H.C.C.I. mode for light to moderate loads then for higher loads you would only need to delay the injection timing to have it operate in normal diesel C.I. mode.

For gasoline H.C.C.I. is obviously the same but introducing a spark plug and lowering the compression would allow it to operate on conventional spark ignited mode.

Any thoughts, ideas, flaws, or suggestions?
 

Answers and Replies

  • #2
Mech_Engineer
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How would you compress the intake charge?
 
  • #3
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an external larger compression chamber sharing the same crankshaft.
 
  • #4
Mech_Engineer
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an external larger compression chamber sharing the same crankshaft.

When you say compression chamber do you mean a large piston?
 
  • #5
563
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Considering how much time it takes to fill cylinders through the conventional intake port even when supercharged, I'm not sure you'll be able to accomplish what you're thinking in an efficient manner.

How would this be better than the conventional induction system? It does all the things that you're wanting now.
 
  • #6
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Piston or rotary. It would be better for efficiency because it fires ATDC and could run in HCCI mode. Used with gas it would be way more efficient because it wouldn't need to be throttled and ran in a vacuum. Emissions with gas and diesel would be way lower because of complete combustion at lower temperatures. Supercharging would be mechanical. It would only require a larger compression cylinder. A good portion of the aeobatic heat from compression would be lost when the compression charge was released into the combustion chamber. Again I appreciate any criticism
 
  • #7
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Mender, I'm talking compression injection pressures of around 50bar
 
  • #8
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Considering how much time it takes to fill cylinders through the conventional intake port even when supercharged, I'm not sure you'll be able to accomplish what you're thinking in an efficient manner.

How would this be better than the conventional induction system? It does all the things that you're wanting now.

You can't change compression based on load
 
  • #9
563
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Mender, I'm talking compression injection pressures of around 50bar

Okay, how much energy would that take compared to a conventional compression stroke?
 
  • #10
563
2
You can't change compression based on load

But it does; I'm sure you're familiar with dynamic (effective) compression ratio.
 
  • #11
563
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Used with gas it would be way more efficient because it wouldn't need to be throttled and ran in a vacuum.

Are you remembering that the induction stroke has losses because of the pressure differential between the induction system and the crankcase? Or are you proposing to run the crankcase in a vacuum as well?
 
  • #12
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Okay, how much energy would that take compared to a conventional compression stroke?

That's why I'm here lol
 
  • #13
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But it does; I'm sure you're familiar with dynamic (effective) compression ratio.

True but not by the extreme amounts you could with this. It's also WAY more simple than trying to do it with camshaft timing etc. It's also more precise on a cycle to cycle basis.
 
  • #14
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Are you remembering that the induction stroke has losses because of the pressure differential between the induction system and the crankcase? Or are you proposing to run the crankcase in a vacuum as well?
This is not a problem with a rotary. This is my actual idea
 
  • #15
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This would use no camshafts no mechanical valves etc.
 
  • #16
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Okay, I'm getting the idea. I'll cogitate for a while!
 
  • #17
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Okay, I'm getting the idea. I'll cogitate for a while!

lol thanks I don't want to share too many specifics because I'm in the process of getting a patent. It would just suck to spend all this money and my idea have some huge flaw I didn't think of. I've started the process but that only goes so far..
 
  • #18
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I'm not that concerned because the big idea is how I get the compressed air from one chamber to the other..
 
  • #19
t92
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I was wondering about a split cycle engine too this week.

In this version the piston compresses say 2/3 of the air into a separate reservoir, then a valve shuts and the piston continues to compress the remaining third of the air to 1/6 of the cylinder volume, i.e to half the remaining air's volume.


The fuel is injected now at tdc and a spark ignites it. The air expands and the inertia of the flywheel recompresses the burnt gas also into the reservoir.

A second expander piston then is driven from the air in the reservoir. It is a bit like a turbocharged lenoir cycle. Trading peak temperature for a much more fully expanded and cooler exhaust gas.

Is this comprehensible and does it sound sensible?

(It's similar but not quite the same as one of the operating regimes of the scuderi split cycle engine.)
 
  • #20
18
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I was wondering about a split cycle engine too this week.

In this version the piston compresses say 2/3 of the air into a separate reservoir, then a valve shuts and the piston continues to compress the remaining third of the air to 1/6 of the cylinder volume, i.e to half the remaining air's volume.


The fuel is injected now at tdc and a spark ignites it. The air expands and the inertia of the flywheel recompresses the burnt gas also into the reservoir.

A second expander piston then is driven from the air in the reservoir. It is a bit like a turbocharged lenoir cycle. Trading peak temperature for a much more fully expanded and cooler exhaust gas.

Is this comprehensible and does it sound sensible?

(It's similar but not quite the same as one of the operating regimes of the scuderi split cycle engine.)

If you inject the fuel at TDC how will you keep it from igniting before the spark.
 
  • #21
t92
8
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If you inject the fuel at TDC how will you keep it from igniting before the spark.

the compression ratio will be quite low between 2 and 4 perhaps. So I don't think it will be hot enough to knock. Or am I missing something?
 
  • #22
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the compression ratio will be quite low between 2 and 4 perhaps. So I don't think it will be hot enough to knock. Or am I missing something?
When you say 2 to 4 I am assuming you mean 4 to 1. If thats the case it wont be enough for combustion. you will need around 150 psi or more.
 
  • #23
t92
8
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When you say 2 to 4 I am assuming you mean 4 to 1. If thats the case it wont be enough for combustion. you will need around 150 psi or more.

yes 2:1 - 4:1 compression, but coupled with 6:1 - 12:1 expansion. I think a little compression goes a long way in improving the thermodynamics. Particularly when you have a very large expansion piston, as in this design.

I did do a spreadsheet based on the adiabatic relationship. Okay the next bit I'm a bit less sure about; but I thought I read that if fuel is burnt in a unit of air at atmospheric temperature, such that all the oxygen has been used up, that the air would have enough energy to expand to 3 times its volume. Using this as my guide I made the assumption (I hope correctly!) that compressing a gas to 2:1 and then igniting it should be equivalent to getting a unit of air and then compressing it to 1/6 of its volume. Working back with the adiabatic relationship assuming no losses! [and spherical cows experience no air-resistance ;-)] the resulting ratio of the temperature of this gas at the begining (300K) and at the end (compressed 6:1) gave surprisingly reasonable theoretical maximum efficiencies even at such a low compression ratio. I think it may have been between 40-55%.

Anyway I'd be intrested to know if anyone thinks this methodology is sound.
 
  • #25
I just joined this group.. I've never studied formally but I've made some piston Brayton cycle and Ericsson cycle engines with an extended piston... I made the engine adjustable and played with compression / expansion ratios... I've studied the original engines of Brayton.. All of Braytons original engines were piston and some had varied ratios expander to compressor between 1 to 1 and 2 to 1... most had a working pressure of about 65 psi between the compressor and expander... The piston Brayton engine was fairly efficient but they all had a cooled expander chamber / piston so IMO that lowered the efficiency considerably. I hope this answers some of your questions..
 

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