Is it possible to make a hot bulb Diesel engine from a petrol lawn mower engine?

[Simon Robinson 4Participant @simonrobinson4]

[Simon Robinson 4Participant @simonrobinson4]

Hi

This is purely experimental, but would it be possible to convert a Briggs Stratton petrol mower engine into a hot bulb Diesel engine?

My idea would be to make a new cylinder head containing a hot bulb, But what kind of injection system were used in the original hot bulb engines? Is making an injector very difficult in terms of machining etc?

[Bob RodgersonParticipant @bobrodgerson97362]

Why not just make a diesel engine? Are they a side valve? if they are you will have some major redesigning to do.

[Clive FosterParticipant @clivefoster55965]

In principle it should be possible but the engine may end up being too small to run properly. Hot bulb engines in general are hefty beasts with large reciprocating and rotating masses helping to keep them running at their low operational speeds, typically in the 200 rpm range. Plenty of meat in the cylinder heads to help keep the bulb temperature stable. Something based on a Briggs & Stratton may well not have enough inertia to keep going unless the set up is very precise. Which to a degree misses the point of a hot bulb engine which is supposed to be able to run under almost any remotely suitable operating conditions. You could hang a hefty flywheel on but I'm unconvinced as to the durability of a Briggs & Stratton crank and crank case if you do.

Injection system can be very low pressure as fuel is injected towards the end of the inlet stroke or very beginning of the compression stroke. Simple air blast squirting fuel through a nozzle should be fine. Valve timing might be need some work. No idea how you would establish the volume of the hot bulb chamber which is probably a fairly important parameter in a small engine.

In many ways a hot bulb engine finds its own operating point as a sort of balance between, load, set-up, fuel and other things, probably including phase of the moon. Which is why they do fine under long term running at fairly constant load and speed. Once set-up and going well the just keep on keeping on with engine thermal and technical inertia helping to ride out variations. Try and use them for variable speed variable load applications and things can get interesting. Some of the two stroke variations were quite capable of self reversing at lower speed should they feel so inclined. Which could be interesting in applications such as the Lanz tractors.

i have horrible feeling that getting it to run reliably and successfully will need a higher compression ratio and more sophisticated control going some way towards a diesel engine rather after the manner of Ackroyd Stuarts work with Hornsby's.

Clive.

[Nigel McBurney 1Participant @nigelmcburney1]

forget it, look at any hot bulb semi diesel or surface ignition engine ,its really heavily built, diesel or kerosene leaking past the piston rings will destroy the sump oil,a lot of these early engines were also two stroke and these will occasionally run off any build up of oil in the sump , if you ever see a large Petter S type run away off the governors you will also run very fast to get away from it.

[Jeff DaymanParticipant @jeffdayman43397]

Hi Simon,

A B&S mower engine is really not the best choice as a starting point for any kind of diesel conversion. Diesels use high compression and the connecting rod and cylinder in a B&S mower engine are lightly constructed in high silicon aluminum. The cyl bore is not lined with steel , the piston and rings run direct in the aluminum. The rod has no bearings, it runs directly on the cast iron crank and the steel wrist pin. The valves are side valve type and the seats will not cope with diesel temperatures or pressures – these engines are low compression by design, detuned to start reliably on any quality of gasoline after long periods of storage. They do their intended job well, I've seen almost worn out ones that have not run for years, sat in the rain and snow, not even a tub over them, that start right up and be capable of mowing.

You could spend a lot of time trying to convert a B&S mower engine to diesel operation and even if you got it running it may only last a few minutes running as a diesel.

You'd be better to start with a small one cyl diesel as found on some commercial concrete mixers and water pumps, in my opinion.

Just my $0.02. JD

[JasonBModerator @jasonb]

[Cyril BonnettParticipant @cyrilbonnett24790]

Try this instead.

**LINK**

[Nick_GParticipant @nick_g]

Posted by Nigel McBurney 1 on 12/01/2017 19:50:27:

and these will occasionally run off any build up of oil in the sump ,

.

Anybody else seen a diesel in a 'run away' ……….. Fun stuff. (except for the owner)

If it's a manual gearbox you may be able to put it in top gear, foot hard on the brake and drop the clutch to stall it. – If it's an automatic box. ……… Just walk away and leave it to do it's thing.

Nick

[Neil WyattModerator @neilwyatt]

Posted by Nick_G on 12/01/2017 20:49:00:

Posted by Nigel McBurney 1 on 12/01/2017 19:50:27:

and these will occasionally run off any build up of oil in the sump ,

.

Anybody else seen a diesel in a 'run away' ……….. Fun stuff. (except for the owner)

If it's a manual gearbox you may be able to put it in top gear, foot hard on the brake and drop the clutch to stall it. – If it's an automatic box. ……… Just walk away and leave it to do it's thing.

Having a petrol engine 'diesel' is pretty exciting too, I've seen that happen a few times in the days of carburettors.

I can't see why a hot bulb wouldn't work, in smaller engines you need to heat the bulb to keep it running rather than just for starting. Higher compression is a red herring as hot bulbs don't need anywhere near the sort of compression a true diesel needs – they are closer to .a glow plug engine .

Neil

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Compression ratio in the semidiesel is not high compared with the real diesel engine. Compression ratio in the semi diesel engine is difference depending on size of hot bulb to example a big hot bulb in a low compression ratio 8:1 while the small hot bulb need high compression ratio up to 12:1. The injector is not a high pressure injector to atomize fuel. Under idling the fuel is hitting on hot bulb to ignite fuel and under full load, the fuel is direct injected into cylinder ignited by heat of compression. The injector is adjustable from thin fuel jet under load to atomized fuel under idling. The regulator can be vacuum regulated (Rap, Sabb semidiesel), stroke regulator (Wichman semidiesel) or centrifugal regulator to regulate amount of fuel depenting on speed of engine.

[Speedy Builder5Participant @speedybuilder5]

Is that diesel ?? You don't need a spark if the head is hot enough. I had a 1937 Austin 7 which, because the ignition was retarded, boiled off all the water and would not stop when the ignition was turned off. When I opened the bonnet (it was a dark November evening), the cylinder head was glowing dull red. What a mess, the rubber hose had melted, head gasket blown on one pot, etc etc, but once I overhauled the top end, it ran right as rain again.
BobH

[DurhambuilderParticipant @durhambuilder]

There is a good build diary on youtube of a Briggs Stratton Engine with a glass cylinder head. Final instalment (of 17) here:

[steveteeParticipant @stevetee]

Beacuse of the shape of the combustion chamber on a side valve engine , even petrol sidevalve engine tuners struggle to get a high enough compression ratio for their needs. There is always a trade off between compression and gas flow on a side valve. It's hard to see how enough compression could be obtained for diesel type ignition.

[Neil WyattModerator @neilwyatt]

Posted by stevetee on 12/01/2017 21:47:44:

It's hard to see how enough compression could be obtained for diesel type ignition.

See Jens' post above, apparently 8:1 is plenty if you have big hot bulb.

Apparently the OHV B&S are 8.5:1 and the SV are 6:1, so a skim of the head should be plenty with an OHV one, a bit more challenging for the side valve.

Neil

[John OlsenParticipant @johnolsen79199]

Glow plug engines use about 7.5 to 1 compression if I remember rightly, but then they depend on a catalytic action too. Maybe if you made the hot bulb out of platinum! You would have to use methanol for the fuel.

John

[Tim ChambersParticipant @timchambers76147]

Has nobody on here heard of Find Hansen?

[JasonBModerator @jasonb]

I suppose the one in the video I posted runs as the big nail he used for the hot bulb is large enough and keeps the engine running when he removes the blowlamp

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 13/01/2017 07:30:41:

I suppose the one in the video I posted runs as the big nail he used for the hot bulb is large enough and keeps the engine running when he removes the blowlamp

No Jason, as people have demonstrated it's impossible so he's obviously got an electric motor hidden in the crankcase

[JasonBModerator @jasonb]

Posted by Neil Wyatt on 13/01/2017 08:03:40:

Posted by JasonB on 13/01/2017 07:30:41:

I suppose the one in the video I posted runs as the big nail he used for the hot bulb is large enough and keeps the engine running when he removes the blowlamp

No Jason, as people have demonstrated it's impossible so he's obviously got an electric motor hidden in the crankcase

Neil, I think you are getting confused with Diesel Shunters, they (at least yours) have an electric motor inside

[Ian S CParticipant @iansc]

Maybe a hot tube ignition, you could start on petrol, and when warmed up change over to Kerosene / Parafin, or you could use LPG, that would be useful as you could use the gas in a Bunsen Burner for heating the hot tube.

Ian S C

[Howard LewisParticipant @howardlewis46836]

Hot bulb engines were started by heating the bulb with a blowlamp, once running, combustion heat should cause the fuel to ignite next time around. Running on light load, that is often not enough, and the blowlamp needs to be applied.

The 2 stroke Bolinder engines used in canal boats were typical of this sort of operation.

The Drainage Museum, at Prickwillow in Cambridgeshire has a large two stroke twin cylinder which needs a blow lamp applied to each cylinder for starting and early running. Once warmed through, it is self sustaining.

The WaterWorks Museum in Hereford, has a Campbell gas engine which requires constant application of heat to the hot tube to keep it running off load.

A normal four stroke carburetted petrol engine, if the compression ratio is fairly high, will run on, (very roughly) for a while, after the ignition is switched off, if the ignition timing is too retarded. The easy way of stopping, was to open the throttle wide, and admit a lot of cool air.

I would have thought that a side valve engine, with a hot bulb would be have so much dead volume as to be unlikely to have a high enough compression ratio to run without application of external heat.

On the subject of engines running away on their own oil, or accumulated fuel; it is not an experience you would want to choose deliberately! Even if you expect it, the rate at which they accelerate is incredible.

Howard

[Neil WyattModerator @neilwyatt]

Posted by Howard Lewis on 18/01/2017 16:42:47:

The easy way of stopping, was to open the throttle wide, and admit a lot of cool air

Now he tells me – I used to crash into fourth with the handbrake on

Neil

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