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Author Topic: ic engine combustion speed  (Read 7760 times)

Offline peppercorn

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ic engine combustion speed
« on: 17/10/2007 11:02:45 »
2 Q's really:

Is the power stroke ignition in an ICE subsonic or supersonic?

If detonation occurs on a spark ignition - does this happen at a faster rate?


 

lyner

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ic engine combustion speed
« Reply #1 on: 17/10/2007 12:13:27 »
I thought that high octane fuel was needed so that the rate of burning was slowed down to avoid the damage that could be caused by the shock wave when fuel burned too quickly. I used to be called 'pinking' - you could hear it when the engine was laboring in a high gear.
This implies that sub-sonic is ideal but not always the case, I think.
 

Offline peppercorn

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ic engine combustion speed
« Reply #2 on: 17/10/2007 12:33:56 »
I guess I'm trying to discover whether a 'perfect' ice would be designed to operate in a subsonic region (which is what I suspect).  This kind of leads me to a question of whether the burn characteristics of supersonic ignition (say in a pulse jet) is likely to be more efficient than in the subsonic case.  By this I mean is combustion generally more complete at supersonic rates? (Obviously overall efficiency of an engine is far more complex).

Because (as you say) pinking happens through compression (due to several factors - low octane rating, overheated cylinder, timing, etc) before the ideal moment it seems likely that it is happening supersonically (although this would imply diesels would be supersonic ignition - doesn't sound right!).

mmmm, more confused than ever!
 

another_someone

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ic engine combustion speed
« Reply #3 on: 17/10/2007 14:21:08 »
My understanding was that pinking happened not because of too rapid a combustion, but that combustion completed too early (before the piston reached TDC) - and in general, faster combustion (assuming that it was timed to complete not sooner than the piston reaching TDC) was a good thing.

I would believe that a faster combustion will mean a higher temperature, which means more efficient conversion of energy.
 

Offline peppercorn

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ic engine combustion speed
« Reply #4 on: 17/10/2007 17:40:44 »
The question still remains is the burn subsonic or super supersonic?

Maybe the is irrelevant to IC engines.

But, having the fuel/air burnt quickly (specifically, faster than the speed of sound) implies, from what's been stated, higher efficiency (heat engine principle thingy!).  In this theoretical engine the peak pressures on engine components are likely to be far higher - hence more expensive materials and manufacture - so six-of-one/half a dozen of the other.

Question 2: Assuming combustion in all normal ICE's is subsonic (is it, anyone?), has there ever been an effort to make a supersonic configuration?
 

another_someone

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ic engine combustion speed
« Reply #5 on: 17/10/2007 19:31:19 »
http://en.wikipedia.org/wiki/Engine_knocking
Quote
The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts.

If true, then it would imply that the flame front is only at 10% of the speed of sound in ordinary air - I don't know what the speed of sound would be in the hot/compressed environment of the fuel air mix in the combustion chamber, it is clear that the flame front is normally below it.

http://en.wikipedia.org/wiki/Engine_knocking
Quote
A non conventional engine that makes use of detonation to improve efficiency and decrease pollutants is the Bourke engine.

http://en.wikipedia.org/wiki/Bourke_engine
Quote
The Bourke engine was designed by Russell Bourke in the late 1930s, who endeavored to improve upon the Otto cycle engine. Despite finishing his design and building several working engines, bad luck (onset of World War II) and bad health compounded to prevent his engine from ever coming to market. This engine was in pre-production by Hudson Motor Car Company (now know as American Motors). For almost a decade there have been several small groups extolling the virtues of the design and engines in testing. Yet not one has produced comprehensive test results conducted by an independent party to back up their performance claims. The main virtues of the design are its small size, light weight, and low number of parts.

Design features

The following design features have been identified

  • Scotch yoke instead of connecting rods to translate linear motion to rotary motion
  • Scotch Yoke does not create sideforces on piston, reducing friction and vibration
  • Fewer (only 3) moving parts
  • Smoother operation due to elimination of crank and slider mechanism
  • Longer percentage of cycle spent at top dead center and bottom-dead-center for more complete combustion and exhaust scavenging
  • One power stroke per piston, for every rotation from the opposed pistons instead of one every other rotation resulting in nearly twice the power at a given engine speed, compared with a 4-stroke engine.
  • Eliminate the need to mix oil with the fuel as with standard two-stroke engines
  • Prevents the piston ring blow by from polluting the crankcase oil extending the life of the oil
  • Low exhaust temperature (below that of boiling water) so metal exhaust components are not required, plastic ones can be used
  • Extremely fast burn time of the fuel air mixture so the engine can be considered to be a detonation or "explosion" engine.
  • 15:1 compression ratio for high efficiency
  • Multifuel capability - although different fuels will need different compression ratios.
  • Fuel is vaporised by the high temperature in the transfer port, and the mechanical action in the combustion chamber.
  • Lean burn for increased efficiency and reduced emissions
  • The piston is connected to the Scotch yoke through a slipper bearing (a type of hydrodynamic tilting-pad fluid bearing).
  • Piston shape, with higher volume around edges, contributes to complete explosive burn



Engineering Critique of the Bourke Engine

The Bourke Engine has some interesting features, but the extravagant claims for its performance are unlikely to be borne out by real tests. Many of the claims are contradictory.

1) Seal friction from the seal between the air compressor chamber and the crankcase, against the conrod, will reduce the efficiency.

2) Pumping losses, the air charge is compressed and expanded twice but energy is only extracted for power in one of the expansions.

3) Engine weight is likely to be high as it will have to be very strongly built to cope with the high peak pressures seen as a result of the rapid high temperature combustion, and the scotch yoke/triple slipper bearing are heavier than a conventional crankshaft.

4) Each piston pair is highly imbalanced. This will limit the speed range and hence the power of the engine, and increase its weight due to the strong construction necessary to react the high forces in the components. (videos of running engines do not appear to show imbalance)

5) High speed two-stroke engines tend to be inefficient compared with four-strokes because some of the intake charge escapes unburnt with the exhaust.

6) When the charge is transferred from the compressor chamber to the combustion chamber it will cool down, reducing the efficiency of the engine.

7) Use of excess air will reduce the torque available for a given engine size.

8) Forcing the exhaust out through small ports will incur a further efficiency loss.

9) Operating an internal combustion engine in detonation reduces efficiency due to heat lost from the combustion gases being scrubbed against the combustion chamber walls by the shock waves.

10) Emissions - although some tests have shown low emissions in some circumstances, these were not necessarily at full power. As the scavenge ratio (ie engine torque) is increased more HC and CO will be emitted. See HCCI engine
 

lyner

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ic engine combustion speed
« Reply #6 on: 18/10/2007 10:25:09 »
Thanks for that a_s.
 Interesting.
 

Offline peppercorn

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ic engine combustion speed
« Reply #7 on: 18/10/2007 11:11:50 »
Indeed. Very interesting!

The story of the Bourke engine & the guys who carried on his work is really quite a sad tale.  What with Bourke's illness and premature death, plus the following tale (if totally true):

www.niquette.com/books/sophmag/bourke.htm

It's amazing how compelling ideas can sometimes blind us to reality - These guys thought they had a Eureka moment that kept them going for decades.

I don't know about you guys, but I feel that rotational heat engines (turbines, not Wankel) are always going to have an advantage thermodynamically over reciprocation every time.

The reason why reciprocating ICE's are still the mass choice for cars, etc is the closer matching of speed & torque, as well as cheaper manufacture & maintenance, plus probably more rugged.
 

lyner

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ic engine combustion speed
« Reply #8 on: 18/10/2007 23:10:51 »
Quote
www.niquette.com/books/sophmag/bourke.htm
What a fantastically poetic bit of engineering!
Really entertaining.
 

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ic engine combustion speed
« Reply #8 on: 18/10/2007 23:10:51 »

 

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