If you’re into automotive hacks and don’t watch [Robot Cantina], you are missing out. This hack has [Jimbo] taking a break from automotive hacking to butcher a poor, innocent Tecumseh lawnmower to run diesel fuel (or anything else) by converting the motor into a hot bulb engine. (Video embedded below.)
The secret is a long stack of anti-fouling adapters, which are essentially extension tubes that move the spark plug out of the combustion chamber to keep it from getting crudded up in an engine that’s burning too much oil. In this case, burning is what’s happening inside the anti-fouling adapters: by stacking seven of them, [Robot Cantina] is able to create a hot-bulb– volume that stays hot enough between strokes to induce spontaneous combustion of the fuel-air mix.
Hot-bulb engines were popular for certain tractors (the Lanz Bulldog being the most famous) and stationary engines from the late 19th century until Rudolf Diesel’s eponymous invention drove them out of their niche completely sometime after WWII.
Traditionally, a torch would have been used to heat the hot bulb, but here [Jimbo] starts the engine on gasoline with a spark plug at the end of the stack, and disconnect the spark once the hot bulb has warmed sufficiently. Given how rough the engine runs before the ersatz hot bulb heats up enough for spontaneous combustion, tradition seems like the way to go. Even once heated up, the “chaos in the combustion chamber” (probably knocking) is enough to pop the (now disconnected) spark plug from the end of the stack of adapters at one point.
While shockingly an inspection revealed no obvious damage to the engine after the first day’s experiments, this is probably not a hack you would want to use on a motor you intended to keep — or run for very long, for that matter. Practical or not, it is fascinating to know you can go back in time to the hot-bulb era with such a simple modification. Watching this motor pop and snarl while drinking down diesel fuel, acetone, or 190 proof alcohol is a bit like watching the proverbial dancing bear: the point is not how well it dances, but that it can dance at all. If you can’t get enough of it, they made a second video that features further fuel-testing fun, and even a mowing montage.
We’ve featured other [Robot Cantina] hacks that were arguably more practical, like hacking an old Saturn’s ECU to allow for Honda Insight-style lean burning or this DIY dynomometer for similarly small engines.
Just want to say that Tecumseh is an engine not a lawn mower. Neither Briggs nor Tecumseh make lawn mowers.
Terminology aside this is fascinating…
The tube reduces the compression ratio, making it possible to run on diesel without detonation (diesel has a lower octane equivalent number than gasoline). Spark ignition engines can run just fine on kerosene (similar to diesel) if you lower the compression ratio, and this was commonly done during the 70’s oil crisis when gasoline prices went through the roof. All sorts of ersatz fuels were used, including turpentine.
Detonation in a diesel engine is controlled by the fact that the fuel isn’t pre-mixed with air but misted on injection, so the droplets burn on the surface rather than going bang all at once.
An engine designed for kerosene would have a compression ratio around 6.5:1 instead of the 8.5:1 in a typical lawnmower engine, which is relatively low compression still for a gasoline engine. An engine designed to run on 95 RON fuel would be at 10.5:1.
Saab and Talbot modified their engines during the oil crisis to accept kerosene by lowering the compression ratio in some special models. A soviet Lada already had low enough compression that it could run more or less on kerosene or whatever other junk they had for fuel without modification – provided you could get it to start. They typically had dual fuel systems with a valve to switch form gasoline to kerosene when the engine was hot enough.
I always remember an acquaintance buying a Russian Ural motorcycle with the “high-compression export engine” – 7:1 IIRC, anyway it was hysterically low. It ran happily on Shellite (white spirit) which Gravelly mowers used as it had no road tax on it, so a bit of a win.
Which reminds me of the joke a Russian told me many, many years ago: “In West you have many freedoms, but in Russia we have one you do not have. In Russia we can light matches in petrol station. Even if you can get Russian match to strike, Russian petrol won’t burn.”
There is one Soviet motorcycle that I’ve seen on the road, not sure if it’s Ural or something else, that revs so low on overdrive that it sounds like the rear wheel is turning way faster than the crankshaft. It must be doing one bang every five meters on the motorway.
Saab had a prototype engine where the upper half of the block was hinged between the crankshaft and the cylinders. It could vary the compression ratio while running. This could be quite a boost (pun intended) to low end power as you would have a high “static” compression ratio until the turbo was generating a significant amount of positive pressure. Then the static compression would be lowered for knock resistance under high boost.
Also see the Porsche 6 stroke engine patent, very interesting. It has a high pressure and a low pressure combustion cycle. I think it uncovers extra intake ports on the longest compression stroke though.
Oh, and another way to control detonation is to retard the ignition. Normally the fuel is ignited by the spark plug while it’s still being compressed so the charge would be fully lit by the time the piston reaches maximum compression – that’s called spark advance – but with a fuel like diesel this would tend to detonate the charge.
With the hot tube acting as extra volume in the engine, ignition is probably delayed until the piston pushes the fuel mixture all the way into the tube, and the burning fuel shoots out when the piston has already started to go down. That’s probably what makes the pock pock pock sound – it might even ignite so late that the exhaust valve is already open, sometimes missing the cycle entirely.
Tractors were very widely run on TVO – tractor vaporising oil. It was a grade intermediate between kerosene & gasoline/petrol. The tractor was started on petrol & once warm switched to TVO.
An engine built for ethanol should start with 40-60% ethanol in water with some difficulty – that’s about the lowest mixture you can still ignite from the fumes at room temperature. If you run the engine hot enough, even lower percentages should run but at some point the heat required to turn the water into steam is going to be more than there’s available from the burn and there’s not enough energy left over to turn the engine.
I can confirm (for a friend) that 70% alc/vol home distilled you’re not sure about, when injected via windshield washer pump through an orifice made by crushing a stainless steel parker pen, into the intake of an already running 1993 dodge caravan with 300,000km on it, will make said example minivan do a burnout of indeterminate length if you stop when you see a cop down the road
Ethanol water injection is one way to “charge” a gasoline engine because the mist cools the intake and lets the engine breathe more air while providing the extra fuel to burn it. This has been used in some production cars as well.
Nothing like walking around the yard behind a cloud of fine particulates.
Well I’m glad to know I’ll be able to mow my lawn after the apocalypses.
Nothing says the apocalypse has to be messy.
But seriously you could connect this to an alternator and tadahh: DC generator for the apocalypse
That’s actually why I clicked on the article, would it work as a gen?
https://www.qsl.net/ns8o/Induction_Generator.html
Presumably the opposite of apocalypse is “pocalypse”, which is what we’re in right now?
OK, now lets get one of those tiny motorcycle turbos mounted on it. Once that is good and balanced we can see about a DEF system just for the look of the thing, and to keep the HOA NIMBYs happy. Then, we’ll rig an exhaust bypass and a second injector to roll coal when no one is looking. With enough insurance and a sketchy law firm we can sell the whole thing as a kit. Maybe spin up a second company to sell “DEF delete kits” and make money off the gear heads who prefer more NOx.
In all seriousness though, this is pretty cool.
You joke, but I’m pretty sure I’ve seen NOx, turbos and even a supercharged mower engine on the tubes and wider world of web
Colin Furze built a jet-powered bicycle, a jet-powered grill, and even a jet-powered kettle (the Jettle) while burning himself severely at times. A supercarged lawnmower would probably seem a bit tame by comparison, but a jet-powered lawn-mower would be right up his street.
On a push mower?
Back when it was still marginally possible to make enough money in a summer job to pay for college (in this case, 1960) my dad paid a significant part of his CalTech tuition by mowing lawns. He didn’t turbo his lawnmower but he did mill the head down to raise the compression ratio significantly and then ran the mower on a mix of toluene and benzene, because he could get those more or less for free through a family friend. He said you had to run at a fast jog to keep up with the mower, and estimated he could mow lawns 3x as fast as pre-modification. (This was a reel-type cutter with the exposed whirling blades out front.)
This only lasted two summers before the piston failed and he had to get a less enthusiastic job.
Reminds me of a guy I knew on the backside of Kodiak island than rebuilt his chainsaw to run on avgas. It was cheaper than gasoline in his village because the octane rating dropped over time. Once it got to low for the aircraft he bought it cheap for logging. It’d make a 4-wheeler scoot too if you were willing to risk it.
Insulate your extension tube with fibreglass. Much higher temps.. better running.
Wonder if he tried a glow plug?