Propane and glow plugs

Hi everyone

Has anyone built the 1895 Otto Deutz engine with the castings supplied by Heinz Kornmueller or the Engineers Emporium? The plans specify using propane as fuel with a glow plug as the source of ignition. I recently finished construction of my engine but cannot get it to run or even give an encouraging "pop". After researching the properties of propane on the internet, I'm not even sure that it should run (ignition temperature is too high) and yet there is a video of such a set-up running on Youtube. Interestingly it doesn't show it being started, and all other videos show the model running on petrol/gasolene. Am I wasting my time and should I just convert it to perol or is there a trick I am missing?

Howard

You may need a demand valve/carb for the propane and will probably have to keep the glow plug connected to the battery all the time as a slow reving engine like the otto won't keep the plug hot enough on its own.

J

The Glow plug engine uses methyl alcohol as fuel. It works because a mixture of air and methyl alcohol oxidises (burns) in the presence of platinum. This happens faster if the platinum is hot. The process is exothermic it -gets hot-.

Propane does not react in this way. to use propane you need a spark plug! OK!

I think something got lost in translation

Rdgs

Dick

Not so richard, if the battery is left connected to the glow plug you will have a permanent "spark" just like a hot bulb engine, there are several small model engines that use glow plugs and regular pump petrol/gasoline with either standard tanks or vapour tanks so if they work with the vapour from petrol they can work with propane.

I agree that these fuels won't get anywhere near hot enough to keep the plug glowing like methanol thats why I said above to keep the battery connected.

J

Edited By JasonB on 19/03/2012 14:04:41

JasonB I think I would try a simple Venturi and needle bar carb and get a small amount of Glow Motor fuel first if the compression ratio is about right.

You also have to remember transerring propane with out a licence is illegal in the UK.  Some twazop blew himself up doing it. 

Rdgs

Dick

Edited By Richard Parsons on 19/03/2012 16:19:35

I acknowledge I have to keep the glow plug connected to the battery (I've already burnt one out doing this), but what really concerns me is that the ignition temperature for propane (470 deg C) is so much higher than that for gasolene vapour (280 deg C). The glow plug glows a bright red but is not sufficiently hot to ignite our gas hob which also uses bottled propane gas. As the compression ratio of the engine is only about 2.5 to 1 there is not much help from that quarter. But then there is the online video showing it is possible (http://www.youtube.com/watch?v=VV1FuxtwA_Q). I thought I had a reasonable understanding of IC engines but now I'm not so sure. I have the plans for a demand regulator, but I think I might try priming with glow motor fuel first as Richard suggests. By the way, what do you mean transfering propane fuel without a licence is illegal? There are lots of things out there running on propane or do you have a particular technical meaning for the term "transferring".

Regards

Howard

Howard

I might be wrong about this but the video shows the engine running from what looks to be a camping gaz blowtorch with the small pierceable blue tin can. I think those are filled with a butane/propane mix, rather than neat propane. Now from what I can find about butane that has an ignition temperature of 288 deg. C so should behave much the same as gasoline vapour. Once the butane content goes pop the propane bit won't have much choice but to follow suit.

Keith

Keith

The reference I've used (http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html) gives the ignition temperature of butane as 420 deg C so not much less than that of propane, but there does seem some variation with the values from different sources (Wikipedia gives 405 deg C). However, they are all much higher than gasolene.

By the way, I managed to work out the compresion ratio wrongly. It is 5 to 1 so still low but not as bad as I first thought. I think I realise most of this when turning the engine over but I'm sure it would improve with a period of running in.

Howard

Hi Howard

The 288 deg C also came from Wikipedia so they really are confused! - Those are the auto-ignition temperatures at which it should go off in the absence of an ignition source, so running as a true diesel - compression ignition. You won't get there without a much higher copmression ratio. The flash points for butane and propane I've seen given as -60 and -104 respectively, which are the values for ignition WITH a source present - ie the glow plug, so both should be capapble of going off with your set up. I wonder if the problem is the air/fuel ratio, from what I saw butane didn't have a very wide spread on that - didn't look at propane. Might be worth a bit of Googling for the answer. Good luck, at least the video shows it is possible so you're not chasing shadows.

Keith

Keith

That's interesting about the flash points for butane and propane although I imagine they would have to be low given that they are both gases at normal atmospheric pressure. I'm not too sure that a glow plug is hot enough to be a source of ignition for propane, but I'm sure you are right about the sensitivity to the air/fuel ratio. I remember reading an article on gas conversions which said just that. Thanks for the encouragement.

Howard

Not directly relevant to the question asked, but I have run an LPG converted car & all the advice for operating on LPG (variable propane / butane mix according to season) is that LPG takes more igniting than petrol. Spark plug change intervals are reduced - even with Iridium plugs - to maintain ignition efficiency.

Nigel B.

Thats where a demand regulator comes in. Without one you are effectively flooding the intake manifold with propane all the time the gas bottle is connected and this displaces air so you in effect get a rich mixture. The demand regulator will only allow gas into the system on the intake stroke as it needs a vacuum in the inlet to open the gas flow.

Having said that it does not look like the one in the video is using a demand reg.

You may also need to play with the temp of the glow plug and the style, I would have thought you don't want one with an idle bar as this will tend to shield the coil, you may also need a longer/shorter one to expose more of the end.

J

hi howard

i have run quite afew model engines and learnt alot mainly the hard way, i shouldnt worry about your compression, once the engine has ran for a while it should get better you could always try the old trick i do of running the engine in on a electric motor this works well.

as said above you need a demand vavle and i think this is where your problem is if your coming straight off the bottle in the engine this will never work these little engines need hardy any fuel at all.

i had the same problem on my half size gardner until i fitted a demand valve and between the valve and the engine you should fit a gauge to see how much gas is going throngh u want the neddle to hardly move.

i would try to restrain from fitting a spark plug, if you do you will need to fit contact points somewhere on the engine and i think with the classic shape of the otto engine the spark plug and contacts will spoil the look of the engine.

if you dont have no joy with gas i would try methanol or sim with the glow plug, on a early style engine like this i would keep away from ungly looking spark plug

hope this is of help

jonathan

Interesting! we have four propane based cars in our family and they all are very gentle on plugs compared to petrol cars!

The glow plug at a good red heat is above 500C so the ignition temperature should be OK, but remember you can get 'hot' and 'cold' plugs.

Electronic gas detectors work by having two balanced platinum wires as part of a wheatstone bridge. One is lacquered, the other bare and it heats up slightly more in the presence of even tiny amounts of gas, including propane, so i'm sure a glow plug should work.

You do need a regulator though, without one the 'stoichiometric ratio' will be way out, you need only a whiff of gas in such a small cylinder. As I recall it needs to be about 15:1 air:gas by vapour volume.

Neil

Re - Ye Law. In the last centenary sometime in the 80s-early 90s HMG passed a law about transferring Propane from one container to another. To transfer Propane from one container to another you have to be licensed and inspected (there is probably a whole legion of such persons and their regulations who do this.)

I read this in Model Engineer of the period. It was part of a series on building a steam plant for a model boat. The one part which the then editor left out was the filling system for the fuel tank which was for propane. The then editor mentioned the law and said that it was probably illegal to publish the filling system. The series author redesigned and made a new type of the valve system for butane/propane mix (LPG) which was published later.

Stub If your cars run on propane I think you have to change the cylinders. Those gars which are filled from a large tank are LPG which is a mixture of gasses. In hungary the mixture of gasses in LPG varies with the season as it can get very nippy (-25C) over here.

Edited By Richard Parsons on 20/03/2012 08:52:19

I can see I'm going to have to make that demand regulator sooner than I thought. In case anyone else is interested I found the plans on the Florida model engineers site at http://www.floridaame.org/HowTo2.htm .

Thanks to all

Howard

There was also a design for aregulator in Model Engien Builder mag not that long ago, I think it used a more easily obtainable diaphram, will have to dig it out.

J

Hi Howard,

Reading all the responce to your query I think most things have been said.

I made the same engine years ago but much smaller and had the same problem.

http://www.model-engineer.co.uk/albums/member_photo.asp?a=6778&p=122275

I found that the mixture strenth of the propane- air mixture is extreem critical.

A little to rich or lean and the mixture is not combustibel .

At the end I switched to gasoline with spark ignition and that made a smooth running engine.

If you want to run it on gas I may advise you to use a gas regulator followed by a demand valve.

Spark ignition is best, followed by "glow"ignition or hot tube. The glowplug needs to be connected while the engine is running.

Sucess with your experiments

Regards, Johan.

Johan

What a beautiful engine. I haven't painted mine yet, but if it turns out half as good as yours I'll be well-satisfied. I won't go down the spark ignition road with this model, but I suspect I'll keep to spark ignition in the future. I have already built a hot tube engine and I'll be trying the demand regulator on that one as well.

Regards

Howard

Hi Howard. The demand regulator design you have works very well but you may struggle to find a suitable diaphragm and needle valve. When made do carry out a hydro check for leakage.

I have recently written an article describing the use for model engines of a commercial demand regulator, this looks similar to the old 'gas bags' used on early town gas powered stationary engines. One of these fitted to a Propane bottle provides a ready-to-go system which can easily be coupled to any engine. It's not cheap though - around £100!

The article is with David for ME magazine but i don't know when, or if it will be published. If you do need any advance information then please let me know and I can e-mail to you.

All the best, John