Steve Hargreaves - Moderator Posted November 16, 2015 Share Posted November 16, 2015 Having a bit of time on my hands recently & inspired by OS's foray into glow plug ignited petrol engines I thought I'd whip out the spark plug from my Evo 10 & fit a normal glow plug & see what happened...... No-one was more surprised than me when it fired up & ran first go.....I let it warm up a fair bit before removing the battery power to the glow plug & was very happy that it kept running OK.....the revs dropped slightly but nothing dramatic....opening up to full power it was obvious that the engine was running a touch rich so I leaned it out by 1/4 of a turn or so. This made for a nice smooth top end but made the bottom end a tough too lean...opening the throttle had to be done very carefully or the engine just stopped. Doubtless this could have been tuned out with a tweak of the idle needle however. It was also very clear that the engine wouldn't idle at low speed either....once the revs dropped below what I would call a fast idle again it just stopped..... Out came the tacho & oh dear!!......I run a 12x6 on the Evo 10 & get just under 10,000 rpm....with the glow plug I was only getting 8,800 to 8,900 rpm....that's a lot of lost power. At the bottom end it wasn't happy below 2,800rpm where as it will idle all day at 2,100 to 2,200rpm on the spark plug. Pulling the plug out later & the element looked very dull & grey....certainly if I saw a plug like this in a glow engine I would replace it immediately but this may not be relevant in a petrol engine. The plug used was a medium "Fireball" plug so possibly a hot (Enya No3 perhaps?) plug would perform better. The obvious solution of course is to fit the OS G5 plug & see what happens....once they have them in stock at my LMS I'll give it a go & see what happens... So, all good fun but such a drop in power is simply too high a price to pay....for now I'm sticking with the spark plug.... Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted November 16, 2015 Share Posted November 16, 2015 I think you'll find the OS "petrol" plug is a bit special! Methanol will spontaneously combust in the presence of platinum, which is why the wire in glow-plugs is coated in the stuff (and also why they are so expensive!). I don't think that platinum has anything like the same effect on petrol, so I'm guessing the special OS plug uses something a bit different! -- Pete Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 16, 2015 Share Posted November 16, 2015 Not an area of chemistry I have any expertise in, but I seem to recall that platinum, and even dimmer memory rhodium, are both catalysts for petrol ignition. Don't they stick platinum in road car petrol catalysts. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted November 16, 2015 Share Posted November 16, 2015 The drop in revs is probably due to a change in the ignition timing with the glow plug, probably something OS have addressed with their plug to make sure ignition occurs as close to a controlled spark as possible. look forward to the test on the G5 plug. Donald the platinum in a car is in the exhaust catalytic converter to convert any unburn hydrocarbons and CO into CO2, it's not used in the ignition system. Edited By Frank Skilbeck on 16/11/2015 16:52:18 Edited By Frank Skilbeck on 16/11/2015 16:53:01 Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 16, 2015 Share Posted November 16, 2015 What is petrol if not hydrocarbons, it catalysed the reaction, it will do the same in a cylinder head. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 16, 2015 Author Share Posted November 16, 2015 My understanding is that it's simply the fact that the glow plug wire is hot which causes the petrol to ignite. If you think about the mixture in a cylinder...a fine mist of petrol mixed with air & then compressed.....this is a very combustible gas & probably doesn't take much to make it go BANG. In a "normal" glow engine there is a catalytic reaction between the methanol & the platinum that helps with ignition & with keeping the plug hot for the next firing stroke.....this will be lacking in a petrol engine which uses only the heat of combustion to keep the glow plug glowing... I don't know but I suspect that the G5 plug is just a glow plug with slightly thicker wire & designed in such a way that it stays hot for longer....at 9000rpm there is only 0.006667s between firing stokes & even at 3000 rpm only 0.02s so the plug won't have to stay hot for very long before there is nice fireball to heat it back up again!!! Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 16, 2015 Share Posted November 16, 2015 Steve, a catalyst does not provide heat, it is merely an assistant to a reaction which allows the reaction to proceed at a lower temperature and/or pressure than if the catalyst is absent. Now while I am happy enough with the generalities of catalyst chemistry, I now leap into the unknown of the engineer. I recall that this clever OS engine can't be started by hand, needs a starter motor. This strikes me that the motor is under compressed, and will not get enough adiabatic heating for diesel ignition by hand cranking, and relies on either a substantial heat residue in the plug, or a catalyst, or both to aid ignition, and being a bit retarded in timing is generally smooth and nice to use. I visualise this as the way this works, as this is the only way I can see to avoid ignition before TDC. Any engineers out there? Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted November 16, 2015 Share Posted November 16, 2015 Posted by Donald Fry on 16/11/2015 17:44:50: What is petrol if not hydrocarbons, it catalysed the reaction, it will do the same in a cylinder head. Donald, I think we are at cross purposes, you said you thought Platinum was used in Petrol Ignition, it is used to oxidies residual unburnt hydrocarbons and CO in the catalytic converter (which wouldn't burn normally due to the weak mixture) it doesn't cause ignition in the traditional sense. In a methanol glow plug the methanol reaction causes the platinum part of the glow plug to heat up thus helping to maintain the glow and it's the hot glow which causes the ignition, if it was a catalysed reaction then it would still work if the mixture was too lean. Note in the car exhaust the catalytic reaction doesn't become effective until the exhaust is up to temperature. Apologies if my original post had misinterpreted your original post. So while a conventional plug may work in an petrol engine it may be less hot and the ignition is effectively retarded?which causes the loss in rpm, would be interesting to see what the rpm is if the plug is kept energised (hot), didn't Brian Winch do some tests on this a while back in RCME? Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 16, 2015 Author Share Posted November 16, 2015 Posted by Donald Fry on 16/11/2015 18:56:01: Steve, a catalyst does not provide heat, it is merely an assistant to a reaction which allows the reaction to proceed at a lower temperature and/or pressure than if the catalyst is absent. I understand this but I believe the platinum needs to be hot for it to act as a catalyst in the first place...I don't think merely dipping some platinum wire into methanol will cause much of a reaction unless the wire is heated first. So what is actually causing the methanol/air mixture to actually burn in the cylinder do we think? Is the platinum allowing the methanol to oxidise (oxidise to me means react with oxygen which is another way of saying "burn". This reaction, being exothermic, then keeps the platinum wire hot until the next methanol/air charge hits it & the cycle repeats? Or is it the fact that the wire is hot which causes a hot surface ignition? Or maybe a combination of the two? Let's not forget that you can run a methanol fuelled engine using spark ignition too.....no catalyst needed.... In a glow ignited petrol engine then I believe that it is the fact that there is a red hot coil of wire present that ignites the petrol/air mixture; a "hot surface ignition" rather than "spark ignition"....as Frank observes the lack of power may well be down to the ignition point being retarded. The mixture is usually ignited before TDC so that all the burning takes place & the maximum pressure is reached in the cylinder just as the piston passes TDC...this allows the maximum energy to be imparted to the descending piston. If the ignition is retarded then the maximum cylinder pressure doesn't develop until the piston is partway down the power stroke which means less power is produced by the engine... Quote Link to comment Share on other sites More sharing options...
Lee Damms Posted November 16, 2015 Share Posted November 16, 2015 Steve, I've also got a EVO 10GX and thought about fitting an OS petrol glow plug but on getting a mate of mine to take his out of his engine I found that they are much longer than a standard glowplug, I don't know how this compares with the small spark plugs on the EVO 10 length wise.Regards Lee Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 17, 2015 Share Posted November 17, 2015 The cause of the burn (oxidation, explosion, detonation), is the fuel air mix is compressed already, and hence hot by adiabatic heating. It is ready to blow. Apply a catalyst, a hot spark, or in a pure diesel, the heating alone, it blows. Platinum catalysts need to be hot to get our glow plug fuel to burn. Some fuels don't need heat, I think I recall that a mix of oxygen/hydrogen can be ignited by platinum at room temperature. No home laboratories please. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 17, 2015 Author Share Posted November 17, 2015 Interesting stuff Lee.....I wonder if the G5 is a bit longer to get it to heart of the combustion chamber to better ignite the fresh charge & possibly benefit from the heat of combustion in staying hot? If it protrudes into the combustion chamber then this will also have the effect of raising the compression ratio slightly which might help. From memory the standard spark plug simply sits at the top of the combustion chamber with the earthed electrode just protruding slightly into the chamber itself.... @DF so it is simply the presence of hot platinum acting as a catalyst that causes the methanol to burn rather than the heat of the wire coil itself actual igniting the mixture? Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 17, 2015 Share Posted November 17, 2015 If the wire is not hot enough, no bang, but using a catalyst, in this case platinum, we get the job done at dull/cherry red temperatures. Use a spark plug, and you need white hot spark to do the job. Same job getting done, but in much friendlier working conditions. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted November 17, 2015 Share Posted November 17, 2015 I may be wrong but my gut feeling is that the ignition timing of a "chemical" glow ignition system is never going to be as precise as an "electric" spark system. The timing of the spark system can be adjusted electronically so that the explosion take place at exactly the right time to suit the conditions. A chemical system will explode when the flash point is reached, which will depend on all sorts of variables (temperature, pressure, RPM etc). Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted November 17, 2015 Share Posted November 17, 2015 Posted by Steve Hargreaves - Moderator on 17/11/2015 12:45:45: @DF so it is simply the presence of hot platinum acting as a catalyst that causes the methanol to burn rather than the heat of the wire coil itself actual igniting the mixture? The Methanol vapour in contact with the platinum wire will react much quicker due to the catalytic action and in doing so heat up the wire which is what helps it too maintain it's temperature so it can then ignite the fuel next time around. From wikipedia " The fuel ignites when it comes in contact with the heating element of the glow plug. Between strokes of the engine, the wire remains hot, continuing to glow partly due to thermal inertia, but largely due to the catalytic combustion reaction of methanol remaining on the platinum filament. This keeps the filament hot, allowing it to ignite the next charge, thus sustaining the power cycle." Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 17, 2015 Share Posted November 17, 2015 Not too sure if Wikipedia is that accurate with that description. First sentence, ignition at or near the top of the compression stroke, no problem. Second sentence, thermal inertia, no problem., but second part of sentence, methanol, after ignition, stays reacting on th element, in the exhaust cycle, and through induction cycle. When I hear someone explain the law of physics for why methanol should hang about like this I will accept the entry.. Thermal emerita I can buy, start a motor, immediately disconnect the battery, and open it up. What happens next. No heat in the system. May be wrong, but I would like a reasoned explanation first, as to why the catalyst has a.significant effect at any time other than at ignition Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted November 17, 2015 Share Posted November 17, 2015 I can't confirm the Wikipedia entry is correct, but assume nobody else has pointed out it is incorrect. But if you take the analogy of the catalyst in your car exhaust system, what happens it speeds up a reaction which would take place normally (but take way too long), but the energy released in the reaction is still released. So the analogy with the glow plug would be you get ignition and lots of heat, then on the next induction cycle as the plug is cooling down the new methanol starts reacting on the plug element before you get to the full mixture combustion, this local reaction in effect keeps the wire glowing red hot, sufficient to ignite the mixture as it is compressed and heated up. Some more info here and here Quote Link to comment Share on other sites More sharing options...
Don Fry Posted November 17, 2015 Share Posted November 17, 2015 Frank, you are correct, you describe heat inertia, but the quote you make from Wikipedia, states that the reason the plug glows through the cycle is largely due to the catalytic combustion of methanol remaining on the filament coil. I know of no law of physics why this should be so after ignition. "Largely due", in my book, is a majority reason, or at least a substantial reason. I wait for a physicist to put me right. Wikipedia is no more authoritative than a peer reviewed science textbook, or paper, and I have seen some rubbish published in those. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted November 18, 2015 Share Posted November 18, 2015 A question for Lee - how does your mate's OS gas/glow engine run? The reason I ask is that I've seen one or two comments on RCGroups that imply gas/glow is not without it's problems, even on OS's purpose designed unit. Quote Link to comment Share on other sites More sharing options...
Lee Damms Posted November 18, 2015 Share Posted November 18, 2015 Hi Bob, Unfortunately I don't yet he brought it to the field brand new in the box. When I next see him at the field I'll ask. I don't know anyone else that's got oneRegards Lee Quote Link to comment Share on other sites More sharing options...
Clive Hall Posted December 22, 2015 Share Posted December 22, 2015 Platinum has no need for external heating to become hot in the presence of alcohol and air. Years ago I worked in a laboratory and had access to platinum wire, much finer than is used in glowplugs. One day I held a cold piece of this wire in the neck of a bottle of alcohol and . . . it glowed cherry red. It was thin enough for the heat generated by catalytic action to produce the visible temperature rise. Doing the same with a wire of the thickness used in glow plugs would not give a visible effect as the ratio of surface area to mass is much smaller in the thicker wire. The explanation is that at molecular level there are sites (cavities, if you like) on the surface of platinum which can accommodate alcohol and oxygen molecules where they react releasing heat. The products of combustion no longer fit the cavities and are rejected leaving them free for another reaction. This is how the catalytic action works. Part of the function of glowplugs relates to the preheating of the wire, either by battery for starting, or by combustion in a running engine, but the catalytic effect is also significant. Obviously with petrol the surface sites on the platinum do not accommodate the petrol/oxygen molecular pair so the glowplug has to rely on the heat effects alone. Also significant is the temperature rise due to compression of the charge in the cylinder. Car diesel engines use glowplugs to provide a hot spot in the cylinder head so that air heated by compression has some help in igniting fuel when it is injected. A glowplug engine is really a diesel with continuous hot-spot help; a fortunate result of the behaviour of platinum and alcohol in the presence of oxygen. Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted December 22, 2015 Share Posted December 22, 2015 Thank you Clive, for confirming my comments right at the start of this thread! I, too, have witnessed methanol vapour combust in the presence of platinum, without need for heating. It was during basic safety training for chemistry "O" levels (yes, that long ago)! However, as you say, it does need to be vapour and fine wire to see the glow. Methanol burns with an almost invisible flame, as one or two modellers have found out when their flight boxes have set alight without them realising it! However, it is the VAPOUR that burns, not the liquid. To work with petrol, I still maintain that the glow-plug must be a bit different from a normal methanol plug. It may just be thicker wire, though I believe petrol engines tend to run hotter than methanol fuelled types, so thermal retention should be less of a problem. -- Pete Quote Link to comment Share on other sites More sharing options...
Phil Claridge Posted December 23, 2015 Share Posted December 23, 2015 thank you clive, for a neat clear explaination of the process. i often wondered how it worked Quote Link to comment Share on other sites More sharing options...
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