LOM Piston Engines

In this series of articles I will try to present everything I have learned about LOM piston engines, having operated and maintained them since the 1990s. I'll start with a general discussion then provide one article/page for each engine system. - Ignition, fuel, oil, prop, mounting, exhaust etc.

LOM 332Sc installed on Jungmann N28BU

General
LOM Piston engines were manufactured in Prague in the Czech Republic from about 1960 onwards. Over the years production has started and stopped numerous times but spare parts supply and overhaul capabilities continue today. There are always rumors that production is about to restart, but it who knows? 

http://www.pistovemotory.cz/en/

LOM is a large enterprise. They manufacture or overhaul piston engines, jet engines, helicopters and fixed wing aircraft, particularly MIG, Mil and Let aircraft. For this reason, LOM can be a challenge to communicate with. Imagine trying to buy an AN hex bolt from Northrop-Grumman and you will get an idea of what you sometimes have to go through. On the other hand, there are many small independent aviation maintenance organizations throughout the former Eastern Europe that have expertise in these engines.

Developed from the earlier Walter piston engines, the LOM come in four basic models: Six cylinder, four cylinder, with supercharger and without. The first digit in the model number digit is the super charger configuration. 3 = supercharged, 1 = normally aspirated. The last  digit indicates the number of cylinders: 7 = six cylinders, 2 = four. Hence a model M332 is a 4 cylinder supercharged model. an M137 is a six cylinder normally aspirated model. The logic behind this is not obvious, 

The model numbers also have suffixes, most commonly Sc, A, C and AK. The earliest engines are marked Sc, Other letters indicate aerobatic capability (continuous or intermittent) etc. This table provides a guide to the naming conventions: http://www.pistovemotory.cz/en/engines

Aerobatic capabilities are provided by two built-in gravity valves that control the scavenging of oil from the engine back to the tank. Amusingly the extended TBO feature is just a piece of paper. A change to the operators manual states that if run on exclusively Aeroshell ashless dispersant oil or equivalent, the TBO is automatically extended from 1400 to 2000 hours. 

Philosophy and reliability
I spend quite a bit of time on the phone and via email trying to help people with LOM piston engines. They generally fall into two categories: They operate the engine exactly as prescribed by the LOM manuals and enjoy such perfect reliability that they seldom even bother opening the cowling OR - they assume that LOM don't really know what they are doing and operate the engine like the local A&P/Lycoming expert thinks. If I had a dollar for every time I have heard "Well my buddy builds race car engines and he says . . . ". The moral is clear. Operate it by the LOM book and you will enjoy thousands of hours of reliable flying. Run it like a Lycoming with the mixture full rich, no valve adjustment and Lycoming torque values and you are in for a world of pain.

There is a difference in Philosophy between (former) East block engineering and American/European engineering and that is repeatability. If you buy a part from Lycoming, a piston pin say, it will arrived polished and ready to install. You can expect a perfect fit every time. LOM engines on the other hand are hand built. During assembly each part is measured, adjusted and custom fit to ensure a perfect result. As an example of this philosophy, you do not simply order a cam cover gasket. You must fit/torque the cam cover, measure the gap between the cover and the cylinder head and then order a gasket of the correct thickness. They come in ten different thicknesses. Here again you can see where slapping on a replacement part and tightening the nuts is just not going to work on these engines. People sometimes report oil leaks from the engine. Now you can probably guess why!

Design
LOM piston engines are of a significantly more modern design that the typical Continental or Lycoming engine. Amongst other features they have:

• Automatic advance and retard ignition systems to promote easy starting and high power output
• Multipoint sequential fuel injection as found on new cars
• Automatic altitude compensation
• Automatic stating enrichment
• Overhead cam
• Baffling installed from the factory resulting in a ready to install engine
• Built-in unlimited inverted flight operation with no external valves or plumbing
• Optional supercharger for aerobatics, shorter takeoff and high altitude operation
• Quiet, low current starter motor
• Slim profile ideal for tandem seat aircraft.
• Light weight
• Very smooth operation

But the engines are not perfect. Among the disadvantages of LOM piston engines are:

• These are low cubic capacity, high RPM engines. The M332 for example can theoretically produce 160 HP but it does so at 3,000 RPM. This requires a pretty small propeller.
• The clutched supercharger means that you can choose different power configurations (and fuel consumptions) in flight, but then you really need two propellers. The prop becomes even more of a compromise than usual.
• Maintenance of the engines requires strict adherence to the manual. There is no "Bolt it on and go"
• At every fly-in you attend someone will ask "What type of Ranger is that?" and then go on to expound on how "you will never get spares for that". (At the time of writing, LOM spares are easier to get than Lycoming. email the factory, wire them some money and in a few days they turn up by UPS.)
• Because this is an overhead cam engine, removing a cylinder is not an easy task at all. The entire camshaft housing, the fuel injection pump and all the baffling must be removed before even one cylinder to be pulled. This is a major operation!

Overall, pros and cons considered, I would not want to give up the LOM engine in N28BU. It is smooth, reliable, looks good and provides a nice experience for front seat passengers. I cannot climb as fast as a Lycoming Jungmann but then again I can climb at least twice as fast as a Rangsdorf or Altenrhine original.

The Fuel System
The heart of the LOM fuel system is the LOM 5051 fuel injection pump.


The pump has three main sections: A low pressure vane pump with regulator, a pressure sensing capsule, and a high pressure pump.

Looking at the picture above you will see that fuel from the tank enters a fitting attached to a black cylinder running across the top of the pump. This is the low pressure fuel pump. Its job it to create a steady pressure of around 4 or 5 PSI and to deliver that fuel via the steel pipe closest to the camera into the high pressure section. The low pressure pump has a built in filter and pressure regulator which are under the acorn nut that secures the inlet fitting.

To the far right of the pump pictured above are the pressure sensing aneroids. The aneroids expand and contract depending on the manifold pressure, sensed via the small fitting on the top of the chamber.


Pressure sensing aneroid capsules

The aneroids press against this plunger which instructs the high pressure pump exactly how much fuel to inject into the engine.



There are three further mechanisms by which the air/fuel mixture is controlled: The aneroids are also sensitive to atmospheric pressure (they measure the difference between manifold and ambient pressure) so as altitude changes, the mixture is automatically adjusted. There is also a manual override. On the right side of the pump a mixture lever is provided with a graduated scale marked '+', '-' and 'N'. 'N' is the normal position. The system is typically left in that position allowing the pump to automatically regulate fuel flow. In most cases the manual override is never used and in fact it is often permanently wired in the 'N' position.



Finally, on examining the mounting flange where the pump is mounted to the engine, a small oil pressure sensing hole will be observed. The pump uses this to automatically enrich the mixture when the oil pressure is very low as when starting the engine. This acts like a manual choke providing the engine with extra fuel to help starting.



And that brings us to the high pressure section. Imagine four (or six)  medical type syringes stacked side-by-side. The outlet of each syringe is connected to a fuel injector nozzle in one of the four (or six) cylinders. Pressing against the plungers of the syringes there is a gear wheel on the surface of which is a cam. As the wheel rotates the plunger of each syringe is depressed one after the other by the cam in the correct engine firing order. Fuel is injected at +/- 100 PSI.

Now to complete the picture, that gear wheel with the cam that operates the syringes is mounted on a sliding shaft and is pushed closer to, or further from the syringe plungers by the aneroid capsule plunger. The higher the manifold pressure/atmospheric pressure, the closer the gear comes to the syringes and the more fuel is delivered to the engine.

Clever stuff!

This YouTube video shows a cut-away pump from and LOM332 in action



and here the operation of a disassembled pump is shown.



The ignition system
Ignition is provided by two magnetos in the traditional manner. The magnetos are manufactured by the PAL company but under license from Scintilla and are closely related to the American company Vertex which still makes magnetos popular in auto racing and in VW aircraft conversions. Many Vertex components, certainly points, condensers (capacitors) etc are interchangeable.



Engine static timing can be set by loosening the band clamp and rotating the magneto body as desired.

The magnetos used on LOM engines have automatic advance/retard. The initial, static timing is set to 7° before top dead center. At about 1,400 RPM, the advance mechanism operates and the timing changes to +/- 24°. In practice, the timing is not set in this way, however. The manual simply states that the ignition should be adjusted until an exactly 25 RPM mag drop is observed at maximum RPM on the ground. More than 25 RPM means the timing is too far retarded. Less and it is too far advanced. The timing should be checked at least once a year. The RPM drop seems to diminish over time and need readjustment.

The mags do not have impulse couplings. To aid in starting they are supplied with a mechanical vibrator (shower of sparks). This works well enough but it does require cleaning and adjustment from time to time. A very common modification is the substitution of a "SlickStart" device. Being solid state these require no adjustment and with an operating voltage from 9v to 32v a good start is assured with even the weakest of batteries.


Slickstart solid state ignition booster

Another very common update is substituting NGK spark plugs for the original expensive and hard to find Czech plugs. This requires new wires, plugs and caps as follows:

Spark plugs:   NGK DR8HS
Plug caps:      NGK LB05FP
Wire:              Solid Core 8mm MOROSO Blue MAX

These components are inexpensive and are designed for magneto operation. Here are a few pictures of the conversion process:






Electrical system
LOM engines are supplied with a 24v generator and regulator. (A few examples were produced with 12v electrical systems for the US market, but these are best avoided) Unfortunately the generator weighs about the same as a mid-sized family car. There is a nice alternative though. B&C specialty company sell a very light weight dynamo for the LOM engine. https://bandc.com/product/alternator-for-lom-10-amps-homebuilt/#regulator Order the 12v or 24v version as appropriate. DO NOT order the 30 amp version. I have seen several of these installed in the field and in every single case the crank case assembly had cracked due to the unsupported weight of the larger alternator.

The starter fitted to the engine is small and light weight requiring only a very small battery. (A 10AH sealed lead-acid battery is plenty) The reason that the starter is so small is that it drives the engine through the supercharger gearbox and hence benefits from a 16:1 reduction ratio. This means that to start the engine, the supercharger MUST be engaged. If it is not, the starter will spin but the engine will not turn. The starter engagement solenoid is built into the supercharger housing. In an emergency the starter solenoid can be operated manually. Remove the silver cap from the rear of the starter assembly to expose the end of the solenoid, then press the solenoid armature in towards the engine.



Baffling
Like many in-line engines, the baffling is factory installed, very effective and requires no work at installation. Bolt the engine to the mount and you are done. - No rubber baffling material, no high temp silicone. Everything is already in place and ready to go.

Supercharger
LOM models ending in a '2' have a centrifugal supercharger which is connected to the crankshaft via a 16:1 epicyclic gear drive and clutch (band brake). The band brake is operated with a cockpit control; either a cable, pushrod or electric linear actuator. When the supercharger is engaged, +/- 20 more HP are available (on the M332AK) I typically use the supercharger for starting, takeoff/climb and for aerobatics. For cross-country flight and hopping passengers I generally switch it off. But why? What is the difference between 100% throttle with the supercharger off and 90% throttle with it on? - I don't know. The LOM operators manual says to turn it off when not needed and suggests that the a/c is always parked with it off to ensure the band brake does not stick to the main gear.



Because of the supercharger I like to have a manifold pressure gauge in the cockpit and this is actually my main instrument for setting power, particularly since the Performance Propeller "Almost constant speed" prop I use hardly changes RPM at all with power changes.

Plumbing
In general the LOM engine is quite easy to hook up. The only external connections required are:

• Oil in from the tank
• Oil return to the tank
• Oil pressure to the cockpit gauge
• Fuel pressure to the cockpit gauge
• Fuel from the tank
• Return furl to the tank
• Primer

All of these connections are via metric banjo fittings. Where do you get those? Happily these days metric fittings and adaptors are easy to find and in particular, it is possible to buy metric banjo fittings for "standard" AN hoses. Three useful sources for these are Amazon.com, Jeggs and Phoenix Auto Racing. Here are a few examples to get you started: