Even after these few years my engine has still has not completed its conversion, however it has been installed on the firewall and considerable static metrics have been obtained. The attached image is straight out of the Subaru glossy brochure and serves to illustrate the components within the design. The image indicates considerable complexity however, like most automotive engines, the design includes all the elements necessary to install it into an automobile. For aircraft application much of the ancillary equipment is removed.
The Donor Engine
I bought my engine from an auto wrecking yard. The donor vehicle had been rolled in an automotive accident. The rollover switches had stopped the engine and had provided protection against loss in oil pressure in the inverted state. The wrecking yard demonstrated that the engine was still operational by starting it whilst it was still in the car. The images, to the left and below, illustrates the engine the day it arrived in my garage after the wrecking yard had removed it from the car.
The fluids and dust make it look a very sorry sight but the unit was only three months old, when I received it, and it had reached its demise after 5,000 km, (about 3000 miles). This image is the front of the engine in the auto configuration and will also be the front of the engine in the pusher aircraft configuration. To the left of the engine's front, and hanging off to the side, is the "charcoal canister". This canister is used to catch the fuel vapours rather than let them escape to the atmosphere. Upon start-up. the engine consumes these fumes and purges the canister. This component is not required in the conversion.
To the centre and above the harmonic damper is a cover which is hiding the alternator, power steering pump, and the air conditioner compressor. All three are removed including the 90 amp alternator which weighed in at 15 lbs. A lighter 50 amp unit is installed later in the conversion.
The Raw Engine
Peeling back the layers of automotive regulation and EPA equipment, reveals an elegant potential power plant. The white tufts on the top of the engine are white tissue paper blocking the apertures against dirt and debris. The bronze coloured flywheel is the flex plate and is later removed and replaced with an aluminium flywheel with starter ring. The second view, to the lower right, is from the front of the engine and includes the timing belt cover. This cover weighs approximately six pounds and will be removed. The black object to the right and at the rear is the oil filler neck. This is a little bit of an odd shape suited to operation in the car but will probably be replaced by something more suitable for use in the aeroplane. There is a small amount of surface rust on the harmonic balancer that will easily be removed with some belt motion.
The engine is only 22 inches wide at the widest point which makes this engine considerably narrower than the Lycoming it replaces. This results in a more aerodynamic cowling. In this configuration the engine weighs 276 lbs which is lighter than the 0-360. After addition of the PSRU and cooling system the overall weight increases significantly.
The picture on the left show the engine with both the
cooling manifold and the two induction runners with fuel injectors and
fuel rails attached. The fuel rails are latter removed and substituted
with aluminium fuel rails to reduce weight and complexity. Many of the
fuel lines relate to the fuel purging system which will be omitted in this
conversion. The red caps are the plastic tips for the electronic fuel injectors.
The image to the lower-right illustrates the engine with the stock induction manifold and PSRU attached. The stock induction manifold weighed 18 lbs excluding the throttle body and was latter replaced with a home made graphite induction system (illustrated below left). This latter design weighed only three pounds saving a massive 15 lbs. The plenum volume was increased and attention to detail in providing smooth directed air passage through the manifold should increase top end power at the expense of bottom end torque. The need for bottom end torque is non existent in the aircraft configuration which is why the design was modified.
In addition to increasing the plenum volume the runners were increased in diameter by about 10% to increase airflow at the top end. This would have been detrimental to the automotive configuration but it has yet to be determined how this has impacted the aviation configuration.
The Propeller Speed Reduction Unit (PSRU).
The Propeller Speed Reduction Unit (PSRU) is a Ross Aero design. The model displayed in the image was kindly lent to me by Nigel Field. This unit has a reduction ratio of 2.17:1 yielding a maximum propeller speed of 2488 rpm when the engine is turning at it's maximum power rpm of 5400. This propeller speed, in conjunction with a maximum propeller diameter of 70 inches is not a good choice for a high speed aircraft since it forces a very coarse pitch to maintain a good cruise performance. This coarse pitch results in a stalled propeller during the take off roll. Lou Ross preferred this ratio to the lower 1.85:1 offered by the same company simply because the tooth size of the planetary gear were more substantial. By special request Lou Ross made a six planet (the typical unit had four planets) epicyclic unit with a 1.85:1 ratio. This arrangement is presented below as the complete engine package, less coolant radiator. This configuration yields a maximum propeller speed of 2920 rpm at the maximum 5400 rpm engine speed.
Rudimentary calculations for top speed resulting from use of retractable undercarriage and a 230-hp engine yield a top speed of 250 mph. The 2920-rpm propeller combined with a 250 mph aircraft yield a propeller tip speed of mach 0.855 using a 68 inch propeller. This tip speed is just about optimum and will achieve the best compromise between take off roll and top end performance for a fixed pitch arrangement. The choice ratio of 1.85:1 is optimally matched to this power plant.
The graphite induction manifold is clearly visible in this photograph. This construction results in a fifteen pound saving and increased airflow to each of the six cylinders. The profile is almost identical to the stock cast aluminium device that is supplied with the engine.
The exhaust headers is a home brewed design using six equal length pipes into two collector which are coupled using a balance pipe.
Thursday August 31, 2006