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A gas engine question

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AJW03/02/2019 11:26:49
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Currently machining parts for a Centaur gas engine and have started work on the cylinder head.
I have not yet machined the combustion chamber as I would like to get some idea of the compression ratio this type of engine works at, I am guessing it would be fairly low possibly 5 or 6 to 1?
Cylinder is 1 1/2 bore and 2 inch stroke and is designed to run on petrol and gas.

Alan
not done it yet03/02/2019 13:57:56
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One u-toob vid shows only a very thin plate being used as the head. There could not be much removed from that one!

Tim Stevens03/02/2019 17:32:53
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Pump petrol will only stand a max of about 9:1 in full size engines, and then only if the design is good, ignition timing and fuel mixture bang on. With gas it depends more on what gas, exactly. Town gas is very rare in the UK - it was made from coal, and we don't do coal any more.

Hope this helps (but I bet it doesn't)

Tim

JasonB03/02/2019 18:29:50
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Some of the my hit and miss engines are quite low going down to about 3:1 run on Colemans and the Robinson that runs on Propane is around 4:1

Edited By JasonB on 03/02/2019 19:01:44

John Duncker 103/02/2019 18:30:57
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A modern car converted to run on LPG will retain it's normal compression ration of around 9 to 1.

So 9 to 1 should be fine and 8 to 1 conservative.

Howard Lewis03/02/2019 18:39:23
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Current spark ignition petrol engines, run at around 10:1 Compression ratios, to be efficient and to cope with the aftertreatment for low emissioins. My Aygo is Euro 6 compliant, it s 2005 MY Yaris predecessor certainly had a 10:1 C R, and was very fuel efficient!

The Air Standard Efficiency is related to the Compression Ratio; so the latter has tended to increase over the years, in the search for power and fuel efficiency.

The "Town gas" that we get nowadays is natural gas, so is probably likely to contain a butane / propane mix. What the detonation characteristics of that are, I do not know, (possibly low Octane rating?) but someone on here is sure to be conversant with the Octane or Cetane equivalence.

Also, the combustion chamber shape, and spark plug siting, will have an effect, from the point of view of ensuring the mixture being homogenous, (should be far better with a gaseous fuel than petrol ) and avoiding "end gas" which will contribute to a tendency to detonate. Detonation can be very destructive, so is to be avoided, plus the uncontrolled combustion does not improve power output or cylinder temperatures..

Not very specific as regards design, I am afraid.

Howard

XD 35103/02/2019 18:39:39
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You should be able to work out an approximate comp ratio from your drawings .

John Olsen03/02/2019 19:28:34
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Town gas is more likely to be methane. LPG supplied in bottles is likely to be mostly propane, with perhaps some butane. Disposable gas canisters are mostly butane with some proportion of propane. All of these have a higher octane rating than petrol so could run a higher compression ratio. However raising the compression ratio too high tends to mean that the breathing is not so good, due to limited space for valves to open into and the high crown on the piston getting in the way of gas flow. So there is a practical limit quite apart from what the fuel will permit. Smaller cylinders can run a higher compression ratio than larger ones, but this is I think mostly due to the higher heat loss with the greater surface area to volume ratio, so does not give any great gain in efficiency.

All of this is academic for engines like the Centaur, which tend to have low compression ratios. The full size originals were designed to run on the sort of fuel that was readily available, the petrol at the time being pretty much unleaded white spirit. The actual ratio is not critical and the main point of this sort of model is for it to run nicely, so provided the compression ratio is moderate all will be well.

John

Alan Waddington 203/02/2019 19:45:00
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Posted by Tim Stevens on 03/02/2019 17:32:53:

Pump petrol will only stand a max of about 9:1 in full size engines, and then only if the design is good, ignition timing and fuel mixture bang on. With gas it depends more on what gas, exactly. Town gas is very rare in the UK - it was made from coal, and we don't do coal any more.

Hope this helps (but I bet it doesn't)

Tim

Not sure about that Tim, most modern Superbikes are around 13:1 and will happily run on pump petrol.

not done it yet03/02/2019 20:52:48
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Posted by Howard Lewis on 03/02/2019 18:39:23:

Current spark ignition petrol engines, run at around 10:1 Compression ratios, to be efficient and to cope with the aftertreatment for low emissioins. My Aygo is Euro 6 compliant, it s 2005 MY Yaris predecessor certainly had a 10:1 C R, and was very fuel efficient!

The Air Standard Efficiency is related to the Compression Ratio; so the latter has tended to increase over the years, in the search for power and fuel efficiency.

The "Town gas" that we get nowadays is natural gas, so is probably likely to contain a butane / propane mix. What the detonation characteristics of that are, I do not know, (possibly low Octane rating?) but someone on here is sure to be conversant with the Octane or Cetane equivalence.

Also, the combustion chamber shape, and spark plug siting, will have an effect, from the point of view of ensuring the mixture being homogenous, (should be far better with a gaseous fuel than petrol ) and avoiding "end gas" which will contribute to a tendency to detonate. Detonation can be very destructive, so is to be avoided, plus the uncontrolled combustion does not improve power output or cylinder temperatures..

Not very specific as regards design, I am afraid.

Howard

Many diesel engines can be run on LPG or natural gas. They simply need some form of ignition - often a small injection of diesel fuel is sufficient. The modern petrol engine, with higher than about 9:1 will have knock detectors to alter the timing and fuel to air ratio, so I doubt this one would exceed 8:1 at the most. If designed to run on pre-RON rated fuel - the old 70 octane, or less, I would think 7:1 was likely the target, tops.

My petrol/paraffin tractors run on less than 5:1 as the kerosene/paraffin will knock under load at anything higher.

AJW03/02/2019 21:37:55
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Thanks guys, this type of engine is obviously running at a lower CR than I thought!
I shall measure the volume of combustion space to establish what it's maximum ratio could be. The drawings show 'some material' removed from head chamber but it is a bit vague and not dimensioned as such.
Am I to understand that a lower compression ratio is likely to run more smoothly?

Alan
Alan Waddington 203/02/2019 22:48:18
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.Posted by not done it yet on 03/02/2019 20:52:48:

The modern petrol engine, with higher than about 9:1 will have knock detectors to alter the timing and fuel to air ratio, so I doubt this one would exceed 8:1 at the most. If designed to run on pre-RON rated fuel - the old 70 octane, or less, I would think 7:1 was likely the target, tops.

My old Kawi ZX900r circa year 2000 had a CR of 12.2:1 and was carburated, fairly basic ignition, just crank sensor and TPS, so no knock detection, ran happily on regular unleaded.....clever fellows those Japanese engineers

Hopper03/02/2019 23:27:57
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Posted by AJW on 03/02/2019 21:37:55:
...
Am I to understand that a lower compression ratio is likely to run more smoothly?

Alan

Yes. Very much so. Just because modern high rpm car and motorcycle engines will run 9 or even 13 to 1 does not mean a hit and miss banger will like that at all. Back in those machine's day, compression ratios as mentioned in above posts was more in the range of 5 to 1, or even down to 3 to 1.

 

Edited By Hopper on 03/02/2019 23:31:13

not done it yet03/02/2019 23:40:35
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Probably smoother, but less thermodynamically efficient. Fuels have improved somewhat, since hit and miss engines were in regular use!

Nearly twenty year old Kwakas likely only had a useful power band of a couple of thousand rpm or so? They certainly would not like to accelerate hard below 3000rpm in top gear, I would think!

I have a V4 engine that would pull ‘like a train’ all the way down to almost stalling revs. ‘Orses for courses’! Magneto ignition, too.

FMES04/02/2019 06:28:00
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Mazdas Skyaktiv engines run a compression ratio around 14:1 on pump petrol, but the control systems are pretty complicated.

Older 'stationary engines' run 4 - 5 :1.

Remember that pump petrol will light quite happily with no compression at all, just depends on how much power you want from it

Regards

JasonB04/02/2019 07:01:40
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Yes smoother running so the model won't jump about the place.

Also easier to hand start with just a flick of the flywheels, too much compression and it will just bounce back.

Michael Gilligan04/02/2019 07:39:37
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Posted by not done it yet on 03/02/2019 23:40:35:

Nearly twenty year old Kwakas likely only had a useful power band of a couple of thousand rpm or so?

.

dont know ... I've just had a quick browse, to see ^^^

I suppose iit depends upon your personal interpretation of 'useful power band' but this doesn't look too peaky to me: **LINK**

http://www.factorypro.com/images/dynocharts/dyno__stk_to_Micron_SO_K88-1.7-RK_4deg_adv.gif

... Rather more like a high-speed version of the proverbial 'steam train' 

MichaelG.

 

Edited By Michael Gilligan on 04/02/2019 07:46:16

JasonB04/02/2019 08:24:53
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I just had a quick look at the Centaur drawings and looks like the compression ratio works out at about 4.5:1.

Bit of guess work on the head cavity which I came up with 0.64cu.in for. Piston stops 1/4" from the head so at 1.5" bore that s 0.44 cu. in giving a total of 1.02cu.in.

With 2" stroke the swept volume is 3.53cu.in

Therefore total volume 4.55 : unswept 1.02

centaur.jpg

Edited By JasonB on 04/02/2019 08:27:46

AJW04/02/2019 12:57:21
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Thanks Jason!, This morning I used a pipette to measure the combustion chamber volume.
The chamber is at present completely unmachined although I have the spark plug hole drilled it was taped off.
Volume measured 8cc so with a swept volume of 57.8cc it works out to 7.2:1
I shall mill out the deep part of the chamber (up to plug) although I don't believe there is much to come out, more of a clean up operation. I will then bore out for the valves, which should end up fairly flush with the inner surfaces. I will then measure again, after which the shallow tapered part of the chamber will be the only part which could be machined to lower the ratio.
Do you think I ought to aim for something like 5:1? as this is a one way journey!

Alan
JasonB04/02/2019 13:07:37
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Alan, have you allowed for the fact that the piston crown should stop 1/4" from the head?

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