The real problem with diesel engines is that they are NOT light. Diesel engines capable of producing +/- 100 hp weigh +/-100 kg [220 lb] and more. (Don't be fooled by diesel engine weight figures which don't include everything necessary to operate the engine such as; cooling and lubricating liquids, lines, hoses, radiators, turbo and intercooler, alternator, fuel pump(s) air/oil separator - breathers, air mixture boxes, exhaust system, ignition coils/harness, cables, etc !) The diesel engine mount will most probably also be heavier than one needed for a gasoline engine.
This increased total engine weight over a gasoline engine means proportionally decreased payload for the aircraft they are installed into. Compared to the ULPower engine, a diesel of the same power output ready for operation weighs +/-30kg [66 lb] more.
Why do you think diesel is also called "heavy fuel"? Because 100 liters [26.4 US gal] of diesel fuel weighs 19 kg [42lb] more than 100 liters of gasoline.
People tend to think that diesel engines in cars are much more economical than gasoline engines because they use considerably less (in volume) of fuel. A typical figure for an economical gasoline engine would be 8 liters/100km (30 miles/US gal), while a diesel producing comparable power would consume 6 liters/100 km (39 miles/US gal).
In commercial and military aircraft fuel consumption is usually expressed in lb/hr. Using the fuel consumption figures above, 8 liters/hr of gasoline would be 12.7 lb/hr; 6 liters/hr of diesel would be 12.2 lb/hr. Whereas there is a reduction in fuel consumption by volume of 25%, there is only 4% fuel consumption reduction by weight. Compression engines are more fuel efficient because they generally use leaner mixtures and their compression ratio is much higher. Both factors make diesel engines more fuel efficient than gasoline engines. Since the difference in fuel consumption by weight of fuel is nearly the same, this also shows that the specific energy content (per unit weight) of most hydrocarbon fuels is very similar.
The higher specific weight of diesel fuel is certainly not an advantage to the aircraft powered by a diesel engine. Every single airframe manufacturer goes to great lengths to try to increase their aircraft's available payload by reducing the airframe weight. Ultra light manufacturers are jubilant when they can claim a 15 kg [33 lb] weight decrease of their structure without reducing the safety factors.
A diesel powered aircraft with a full fuel load of 100 liters has a decreased passenger payload of +/- 50 kg [110 lb] !! when compared to the same aircraft powered by a ULPower engine. (Note: Kerosene has a specific weight which lies between gasoline and diesel fuel so its' weight penalty is less.)
Many people see the diesel's main advantage in the use of cheaper fuel. However the difference in cost to the public of diesel, Jet A1 and gasoline is purely a matter of government taxes and has little to do with production cost. In many European countries, Germany and Switzerland for example, diesel is the same price as unleaded gasoline. As the public change over more and more to diesel cars and aircraft, governments will increase taxes on diesel fuel to compensate the reduced income from the lower sales of gasoline. In Belgium 3/4 of the new cars bought are diesel because this fuel (used to be) 30% cheaper. The European Union is looking to make the cost of all fuels to the end user dependent on the its' specific energy content and not on the type of fuel.
Another concern is that the aero diesel engines are considerably more expensive to buy than gasoline engines of the same power rating. The diesel manufacturers tell their prospective clients that the increased cost is easily offset by the lower fuel cost - which partly comes from the diesel engine's better fuel efficiency. When you hear this argument, consider two points;
The average recreational pilot flies less than 50 hours a year, and many less than 30. If we presume that diesel power will always have a fuel usage cost advantage over gasoline, the cost savings of diesel engines do not really make much difference to the total cost of flying during a year for this type of pilot when you count purchase cost, maintenance, insurance, hangar and landing fees etc. However, the advantage of being able to have a reserve of 50 kg [110 lb] of extra payload every time you take off (if you use our engine instead of a diesel) is a very important advantage, and not only for the recreational pilot. This weight advantage could easily mean the difference of being able to fly your journey in one go, or having to make an intermediate landing on the way to refuel if this is possible.
In the exhaust emission of diesel engines, the percentage of very tiny carbon particles amounts to 0.2 - 0.5% of the total. This is a potential future problem for aircraft diesel power plants because environmental groups are already complaining about the (small) amounts of soot particles diesel engines produce. Since the number of diesel engines looks as if it will continue to increase dramatically in the near future, so soot particle extractors will eventually become mandatory. Some diesel car manufacturers offer soot extractors as options, others include them as standard because they know it will become compulsory. If/when soot particle filters become compulsory on aircraft, the weight disadvantage associated with diesel power will increase still more..... Soot filters will probably mean an additional (slight) power loss also. Gasoline engines do not have this problem.
An article in New Scientist, (2 November 2002) quoting a study in the Journal of Geophysical Research stated "Petrol engines may not be as harmful to the planet as their more efficient diesel counterparts. A comprehensive climate model shows that the soot produced by diesel engines will warm the climate more over the next century than the extra carbon dioxide emitted by petrol powered vehicles."
Diesel fuel bought at automobile filling stations can cause an additional problem to aircraft users. To keep the paraffin in the diesel fuel from solidifying in very cold weather, additives have to be added. In cold climates, this is done automatically by the fuel refineries. However you could easily fill up with diesel at your local filling station in the summer without an additive and fly into very cold conditions which would mean blocked fuel lines. Even with additives, fuel lines still need to be well shielded or even heated to keep them from blockage because an exposed section could be a source of problems because of wind chill. To keep aircraft operational in cold weather conditions, gasoline engines need few additional precautions, and are therefore one less source of potential trouble for the pilot.
The problem of torque pulse is common to all reciprocating engines but is much more pronounced and a far greater problem with diesel engines. In diesel engine driven systems the problems are compounded by the use of lightweight engines - of the type finding their way into aviation use. This fact has caused the EASA (European Aircraft Safety Agency) to add advisory material to their regulations which draws attention to the need to pay attention to the damaging effects of torque pulse.
The inescapable fact of reciprocating engine operation is that it creates torque pulses that momentarily accelerate or decelerate normal crankshaft operation. The firing order, the firing angle and the number of cylinders cause this. Since the diesel engine does not use spark ignition, and must instead achieve high gas compression to ignite the fuel (up to three times greater than spark-ignited gasoline engines) the pulses are larger than on spark ignition engines. Some degree of misfiring also can cause "pressure spikes", because ignition doesn't always occur precisely when it should and the fuel doesn't burn completely on each ignition stroke.
The torque pulses introduce harmonic excitation forces that are not part of the smooth torque output of the engine and do not add to usable power output. However, they can range as high as ten times the engine's normal operating torque, so they can add substantially to the total amount of torque transmitted through the system and to the rate of wear and damage that results from it.
The phenomena of torque pulse can easily aggravate the relationship between engine and driven equipment in terms of torsional vibration and the resonance that can amplify this vibration to destructive levels.
The use of propellers made of materials which do not easily absorb the torque pulses of reciprocating gasoline engines has often caused serious problems. Torque pulses have lead to the catastrophic failure of blades or retaining hubs. Since diesel engines produce much more violent torque pulses than gasoline engines, unless the diesel engine manufacturer has taken special precautions to limit the torque pulses to levels comparable to gasoline engines, many propellers which presently function well on gasoline engines will not be suitable for a diesel powered aircraft.
Some of the above text has been extracted from:
Since diesel engines have a much higher compression ratio than gasoline engines, the electrical power needed to start the diesel is also considerably more. This will usually mean that a higher capacity (and heavier) battery will have to be installed to have a sufficient reserve of power to make sure the diesel starts especially in cold weather, and the starter motor will also need to be more powerful and heavier than that required by a spark ignition engine. No engine manufacturer includes the weight of the starter battery in the operational weight of the engine.
Ending on a positive note, diesel and Jet A1 fuel does have one advantage that will always remain - it does not explode or burn as easily as gasoline in the case of a crash. There are methods available to fabricate gasoline fuel tanks so that explosions do not occur, but diesel is a safer fuel to have on board in the case of an accident.
As long as diesel engines are heavier and more expensive than gasoline engines for the same power output, we think that almost all recreational pilots of light aircraft have very little to gain for the increased expenditure of buying a diesel power plant for their aircraft. The few certified aircraft which offer the option of piston diesel power plants have only marginal payload capacity when compared to the same aircraft using the traditional gasoline piston engines.
If you take into account all aspects with regard to flying, and not just the fuel consumption/cost factor, we honestly believe our ULPower gasoline engine has a real competitive advantage when compared to any diesel engine of the same power rating for the great majority of owner/operators of VLA, Sport, experimental and ultra light aircraft.