Many people contemplating an auto engine conversion also must contemplate whether they will drive their prop at crankshaft rpm or through a reduction gear setup. Cost, weight, hp, torque, length, reliability and complexity are all factors to be considered.
Direct Drive
At first glance, direct drive setups would seem to offer lower weight than a geared unit. This is true on a rudimentary level in that there is no gearbox. However, a non-geared engine cannot match the power to weight ratio of a geared engine. So if your main concern is to make only enough hp to get the aircraft into the air, a direct drive engine may fit the bill. Climb performance however, may be sadly lacking. Be aware that as much as half of the hp potential is being wasted without a reduction drive unit. You are lugging around a heavy engine which cannot produce its rated hp at the rpm required for the prop to operate at maximum efficiency.
Direct drive's main advantage is low cost. No gearbox, no gearbox cost. This cannot be denied. Certain other factors should be considered on a direct drive setup;
1. Are the propeller thrust loads safely handled by the engine thrust bearings? Remember that there will be something on the order of 600 to 1200 lbs. of thrust acting on the crankshaft and thrust bearings on most light aircraft engine conversions. This is a continuous load.
2. Are the propeller gyroscopic loads safely handled by the crankcase and forward engine bearing. How about when pulling G and with the required prop extension fitted?
3. Will your aircraft still be within safe C of G limits with your heavy little engine fitted?
4. Can you develop the power required at 3000 to 3500 rpm?
On many auto engines, bolting a prop directly to the crank is a recipe for disaster. The thrust and bending loads imposed by a propeller were never considered during the engineering process and the parts may simply not be up to the task. Having the crank snap or the main thrust bearing disintegrate will ruin your day. Some means to properly tranfer these loads to the block is often required. This can be nearly as complicated and expensive as a redrive. Most direct drives need a prop extension/adapter to attach the prop and streamline the engine to fit the cowling. These can be combined with a bearing setup bolted to where the bell housing was on the car.
If your aircraft was designed around an aircraft engine, your direct drive engine installation will likely be heavier for the same installed power. Engine mounts or heavy items such as the battery may have to be modified or moved to correct balance. Do some moment calculations before you start, to see if you can get within the recommended C of G range.
Installed power is the biggest problem on direct drive installations. Maximum power is limited by maximum propeller rpm. Most standard props are limited to around 3000 rpm for safety and efficiency concerns. Some special types might turn to 3500. In any case, this limits the hp potential of most automotive engines considerably, as most engines develop power peak in the 4500 to 6500 rpm range. Using the standard hp and torque formulae below, we can easily calculate available hp and torque at different rpms:
Torque X RPM divided by 5252 = HP
HP X 5252 divided by RPM = Torque
If your engine develops 100 ft./lbs. of torque at 3000 rpm, you are developing only 57 hp. Maximum torque is developed at the point of highest volumetric efficency or VE. Most automotive engines have a VE of between 80% and 95% at torque peak. 4 valve engines tend to have higher VEs right across the rpm range. This is why they develop more hp per cubic inch of displacement. The torque peak rpm is also usually higher on multivalve engines, often making them less suitable for direct drive applications. VE drops off with increasing rpm past torque peak. If you can maintain a high VE at high rpm, you can make a tremendous amount of power. Witness the latest Formula One engines producing nearly 800 hp at 19,000 rpm out of only 146 cubic inches.
We can roughly calculate the hp and torque of a 4 stroke engine just by knowing its displacement by using the following formulae;
1/2 Displacement in cubic inches X RPM divided by 1728 = Engine Airflow in CFM.
CFM X VE = Actual Airflow in CFM
CFM divided by 1.5 = HP
For stock 2 valve engines, use a VE of .8-.85 at torque peak and .7-.75 at power peak.
For stock 4 valve engines, use a VE of .9-.95 at torque peak and .85-.9 at power peak.
These are conservative figures which may be exceeded by some engines. It is better to end up with more hp than less.
As an example, we have a 200 lb., 150 cubic inch, 2 valve engine running at 3000 rpm:
75 X 3000 divided by 1728 = 130.2
130.2 X .8 = 104
104 divided by 1.5 = 69.4 hp.
Working out torque using our formulae, we get 121.5 ft/lbs.
The power to weight ratio is 200 divided by 69.4 = 2.88 lbs./hp.
If HP or torque is known at a certain rpm from the manufacturer or from dyno testing, we can apply the actual VE number into our formulae and work the figures back and forth.
This clearly shows the limitations of developing hp at low rpm. Even if we could achieve 100% VE at 3000 rpm on this engine through careful camshaft design combined with intake and exhaust tuning, it would still only generate 86.8 hp. Raising the compression ratio is the only "free ride" with a fixed displacement on a naturally aspirated engine. This method has fuel octane and detonation concerns, so is only effective to a certain point. There are diminishing returns on CR power increases with increasing CR.
Turbocharging is the only tangible method of seriously improving the power to weight and power to displacement ratios on direct drive engines. By packing more mixture into the engine, we can effectively increase its airflow rates and power. A boost pressure of 7 psi will usually add at least 50% more torque and power to an engine with a weight gain of 25 to 35 lbs. This would take our 69.4 hp wheezer to 104.1 hp theoretically.
Geared Engines
By placing a reduction gear set between the crankshaft and the propeller shaft, we can get both the engine and propeller to operate at their most efficient rpms. The reduction gear also permits us to transfer thrust and bending loads into the stiff engine block and away from the crankshaft. The drawbacks of redrives are of course weight and cost. A typical redrive for a 150-200 hp engine weighs between 35 and 45 lbs. and costs between $2000 and $4000. The redrive usually offers some extra length to help streamline the cowling. Again, C of G problems should be evaluated before starting the project. If you can afford the weight and cost, let's look at the benefits on our example engine:
We can now operate our engine at 5500 rpm for takeoff, where it can develop its full rated hp. We have a 2.2 to 1 reduction gear between the prop and engine so the prop is only turning 2500 rpm. The prop is probably more efficient at this rpm so produces more thrust per input hp, plus it is a lot quieter. The engine torque is multiplied by the reduction ratio when it reaches the prop. We add 40 lbs. to the package weight, bringing it up to 240 lbs. installed.
75 X 5500 divided by 1728 = 238.7 CFM
238.7 X .7 = 167.1 Actual CFM
167.1 divided by 1.5 = 111.4 hp
111.4 X 5252 divided by 5500 = 106.3 ft/lbs.
106.3 X Reduction ratio (2.2) = Prop Torque 234 ft/lbs.
Power to weight ratio = 2.15 lbs./hp
By adding the reduction gear, we deliver 234 ft/lbs. of torque to the prop vs. 121.5, we improve the weight to power ratio to 2.15 lbs./hp from 2.88 and we increase hp to 111.4 from 69.4. Which installation will perform better?
By adding low boost turbocharging to a geared engine, you can achieve even better power to weight ratios and performance. This was the same conclusion that engineers came to on almost all WW2 aircraft powerplants. They were geared and supercharged or turbocharged to achieve the best power to weight ratios possible and gain the best altitude performance possible. The physics make the choice clear.
For further reading on related subjects:
R.F.