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Lohmann Notes

Peter F Vaughan

Reading the comments in recent issues of Buzzing about the Lohmann cyclemotor, perhaps members might be interested in my experiences.

Eighteen months ago I acquired a Lohmann and ,after very useful initial advice from Marque Enthusiast Phillippa Wheeler, I was soon up and running.  Now, after a fair bit of work to the host bicycle, I am registered and legal (thank you for your help, David Casper) and getting about rather more.

As people may know, the 18cc Lohmann engine does not use an electric spark but relies on the heat of compression to ignite the fuel, like a diesel engine.  Unlike a diesel, there is no injection equipment.  The fuel is mixed with air before it enters the cylinder, so the pressure rises on the compression stroke and it all goes bang as soon as conditions are right.  Because of this, the Lohmann has a moveable cylinder head and barrel, allowing the compression ratio to be adjusted on-the-fly as a way of setting the best ignition point.  Without a spark, different fuels behave differently and this seems to have started a myth that the Lohmann needs special fuel.  I therefore decided to experiment to find what works best.  Like many small 2-stroke engines, the Lohmann is lubricated by addition of oil to the fuel which passes all around the engine before ending up in the combustion chamber.  I decided to stick to an oil ratio of 1 in 20 as recommended in the instruction book.  Modern self-mixing 2-stroke oil works OK.  I have tried a proportion of castor oil (the theory being that it is a very 'forgiving' lubricant for fast-revving engines) but apart from being expensive, it doesn't mix very well.

Paraffin works as a base fuel, although Phillippa has found that quality is variable.  This is understandable: it is not sold as engine fuel, and so the oil companies don't have to control the characteristics of auto ignition in a cylinder.

Unleaded petrol (BS 7070), intended for spark ignition engines, has additives to control detonation and this should make it more predictable in its ignition characteristics.  Unfortunately, petrol/paraffin is much more free-flowing than paraffin alone, and I found that the Lohmann will not run unless the throttle is nearly closed.  Even then there is much 4-stroking (mixture too rich).  Above 40% petrol, forget it.  Diesel fuel (BS EN590), on the other hand, is more viscous.  On pure diesel, the Lohmann will run but power is practically non-existent and the throttle has to be turned to the extra-rich position almost permanently.  Interestingly, I have read that heavier hydrocarbons are generally easier to ignite than the more volatile ones because the bigger molecules are less stable when you do get them vaporised.

Using a mixture of the three liquids, I have found a workable mix is 1 part diesel, 2 parts petrol, 4 parts paraffin.  The petrol component makes it run smoother than paraffin alone, and the diesel counteracts the decrease in viscosity caused by adding the petrol.  This blend is mixed 20:1 with 2-stroke oil.  Adding diesel Red-Ex (1cc per litre of mix) seems to make it run better, but more than this makes the engine less flexible and more sensitive to compression setting.  The mixture is legal for road use (See Note) - nearly half has had duty paid, and Customs and Excise allow non-dutied fuels (the paraffin) to be added for engines which will not run satisfactorily on modern fuels.

The Lohmann engine has an adjustable jet needle in the mixing chamber, and mine is currently set with 0.6mm protuding beyond the end of the throttle slide.  With the freedom to use different proportions of paraffin, petrol, diesel, maybe heating oil, different needle positions, there's no way of knowing if all this is anything near optimum, but it is very useable and good enough for me so far.  My little Lohmann always cold starts within 10 yards of pedalling off, and after a mile to nurse it up to running temperature it is a reliable if slow power source.  I have never experienced any overheating problems - backing compression right off and winding the throttle wide open for a few seconds every mile allows plenty of oil-laden fuel around the engine, cooling and lubricating it.

Hopefully all this may be of interest to anyone who has a Lohmann but is put off by the unfamiliar.  Get it out, run it up, and have some fun.  I passed a couple of teenage girls in town on their mountain bikes.  A bit later, they saw me stopped, posting a letter.  "Is that an engine on your bike?" they asked.  "Yes," I said, "Look!"  I walked the bike a couple of yards, then lifted the rear wheel as the engine sprang to life and it whirled round.  "Cool," they said.

First published, October 2002


When this was written, it was believed that the fuel mix described was legal for road use.  However, this was not the case.  The use of mixed fuels was permitted - but only in tractors.

In July 2011, the East Anglian Cyclemotor Club obtained a licence use mixed fuels in Lohmann cyclemotors - so it now is legal to use this fuel - but only if you belong to the EACC.

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