The motor we've all been waiting for!

[Trevor Lyons]

I saw the flat-four side-valve D-motor displayed at Sywell in 2010 and was immediately fascinated by its dinky size and low weight.
http://d-motor1.vpweb.be/Product---Produit.html
Since then, I have become even more impressed: I think that it shows a brilliance of approach and real "thinking outside the box".

Today, people regard side-valves as an archaic pre-war device fit only for lawn-mowers. This is because, to produce more power, engines need to turn faster; and since the volumetric efficiency (or "breathing") of a side-valve engine is poor at high revs, designers of modern engines turned to OHV & OHC valve-gear. However, since a direct-drive aero-engine produces its power at very low revs, it has no need for overhead valves. The boffins at D-Motor are to be praised for ignoring convention and reviving the "humble" side-valve.

In addition to its obvious simplicity, cheapness, compactness and lightness, the side-valve has other benefits for an aero-engine. The combustion chamber design precludes valve-overlap; and the D-Motor makes a virtue of this necessity by providing efficient combustion at low rpm as fresh fuel cannot disappear unburnt down the exhaust. Also, while a side-valve engine becomes inefficient at high rpm, the valve gear (with its stubby tappets and lack of pushrods & rocker arms) can cope easily with high engine speeds. And whereas a dropped valve is catastrophic in an ohv engine (because the valve will almost certainly hit the piston and perhaps cause seizure), valve failure is of minor consequence in a side-valve engine, which could still chug along on its remaining cylinders back to the safety of an airfield.

The flat-four D-motor will be ideal for microlights and VLAs; but I see that D-Motor are also working (with the help of government funding) on a flat-six version.
http://d-motor1.vpweb.be/Recent-information.html
As with the modular Jabiru range, it is very easy to make a 6 out of a 4, since almost all that is needed is a longer crankshaft and crankcase.
It seems to me that the proposed flat-six D-motor may well be the engine that sweeps the board.

The Jabiru 2200 weighs 60kg and produces 85bhp; and its big brother, the 3300 weighs 81kg, and produces 120bhp, 107 continuous.
Clearly then, a flat-six need weigh only 33% more than a flat-four.
(And the Rotax 912 weighs 60kg and produces 80bhp; and its big brother, the turbo 914 weighs 78kg, and produces 115bhp, 100 continuous).

The liquid-cooled D-motor 4-cyl weighs 47kg and produces 80bhp; so (by analogy to the above figures) its big brother, the 6-cyl might weigh only 63kg and yet produce, say, 115bhp or more. In other words, a 6-cylinder D-Motor might weigh barely more than an R912 or J2200; yet would produce nearly 50% more power.
And this is with fuel injection, making carb-heat a thing of the past. If the price is right, it could be the motor we've all been waiting for!

Finally, I remain a fan of reduction gear, to enable the prop to operate slowly and efficiently. If the D-motor (and the Jabiru units too, come to that) were to have a simple 3:2 reduction gear, the prop could be full-size and could extract maximum power while turning less than 2,000 rpm. As it is, both the D-motor and the Jabiru engines must inevitably use small inefficient fast-spinning props; and even so, these engines will probably never be able to reach maximum output, which is 3,300 rpm in the Jabiru's case.

[Trevor Lyons]

A side-valve motor has poor breathing at high rpm, but the simple valve gear offers no mechanical obstacle to high revs; so (in theory at least), provided the D-Motor's crankcase & internals are up to it, there seems no reason why it might not benefit in future from an induction boost via a turbocharger or supercharger. After, all the engine is so light to begin with, the extra weight of a charger would not signify much.

Just imagine: a flat-six turbocharged D-motor weighing perhaps 75kg and producing 140bhp!

[Trevor Lyons]

The revival of another technology as antiquated as the side-valve is discussed at:
http://forums.flyer.co.uk/viewtopic.php?f=1&t=71962
Petrol-paraffin aero-engines may be the way to go!
A turbocharged dual-fuel D-Motor, anyone?

[Tom Sheppard]

You don't need high revs, you need torque and the side valve delivers on that score. It is narrow and light as well. I think it is a very sound proposition.

[bobpanther]

The side valve concept seems good to me as well.

I guess a six cylinder version might increase sales by 30% - after another couple of years of development effort and countless thousands of pounds of investment. And then there’s a second production line to run to set up and run.

Here’s another way to increase sales by 30% - but requires only a week or two of development effort and perhaps one thousand pounds of tooling costs. It even avoids the need for a second production line.

Put some boss’s on the engine sump so the engine can be bed mounted - Rotax 582 style (even Jabiru offer this option).

The market is so large because 582 engines have a life of 300 hours/5 years before major overhaul/change out, and there is no obvious alternative due to power/weight /size limitations. The D-Motor would be the obvious replacement having the following advantages:


same weight (within a few kg)
direct drive (no chattering gearboxes)
4 stroke (less pollution)
cheaper running costs l(more efficient 4 stroke engine, no 2 stroke oil)
uses existing coolant radiators
no nasty narrow peaky engine torque curve
23% more power (more if D-motor up the max rpm ceiling)
30 times? the predicted engine life (for twice? the initial cost)
more environmentally friendly
lower depreciation (per engine hour)
increase in aircraft sales value.

Any idea why D-Motor chooses not to do this?