By Joe Escobar
Help wanted — diesel mechanics needed for aircraft repair facility. It is possible you may be seeing an ad like this within a few years. A certificated diesel engine on an aircraft could be closer than you think. Although several companies have attempted to bring to market a diesel-powered piston engine for the aviation market, few have succeeded. One company, Racine, Wisconsin-based DeltaHawk Engines LLC is making progress toward certifying its diesel engine. Already available for experimental applications, it is working on certifying a two-cycle piston engine that will run on jet fuel. I visited them to learn more about the technology behind the engine. The following article gives some background into this unique powerplant.
The DeltaHawk engine is a four-cylinder two-stroke diesel engine. It is set up in a “V” configuration vs. an opposed setup typical of gasoline-powered engines. The engine is designed to compete with the Lycoming IO-360 engine, so airframes with that engine could be candidates for future conversions. The engine will have two versions — a 160 horsepower, which would be non-intercooled, and a 200 plus horsepower that would be intercooled. The engine is currently in the testing and development stage with certification expected in a few years.
Designing a diesel engine
DeltaHawk began from the ground up when designing its engine. The company felt that path would be better than trying to take an automotive diesel engine and modify it for aircraft applications. Diesel engines used in automobiles are not subjected to the same types of operating stresses those in aircraft would have to endure. Douglas Doers, vice president for DeltaHawk, explains. “A Cummins diesel engine doesn’t have the sustained high-power output that aircraft do. An aircraft engine may run at 80 percent or higher for hours — for however long you have fuel. A 275 horsepower truck engine may need to put out 275 horsepower when you step on it, but when you back down, you are operating at around 60 horsepower on the highway. That is a big difference. Consequently, there is a big difference in the way that you have to design your engine. That is the main reason why automotive versions don’t generally work in aviation. It takes so much development to make it work that when you’re done the cost of that $6,000 automotive engine is now $20,000. So why not start with a new design and have it do exactly what you want? And that’s exactly what we chose to do.”
As we look at some of the different components of the engine, there is one thing to keep in mind — pressure. Diesel engines operate at much higher pressures than gasoline engines. Peak pressure in the cylinders is around 2,000 psi. Because of this, numerous elements of the engine are designed to handle this intense pressure. Let’s take a closer look at some of engine components.
Engine block assembly
One of the first differences that is evident in the engine is that the block assembly is a uni-block construction. The cylinders are not screwed into the block. Instead, they are an integral part of the block with aluminum sleeves installed in each cylinder.
The crankshaft is pretty hefty — it’s about 1/2 inch larger than that on a Lycoming engine. There are two throws with common rods on each throw. The crankshaft is balanced with counterweights. By adjusting the mass of the counterweight, vibration is greatly reduced. Doers explains. “The counterweights kill the first and second order harmonics. In a V configuration, you can do that. One of the reasons we chose a V is the ability to cancel first and second degree harmonics with nothing more than counterweights on the crankshaft.”
The pistons are built to handle the intense pressures of the engine. Conventional engines typically have two bosses on each end with a floating wrist pin. In the DeltaHawk engine, the connecting rod bolts to the wrist pin, allowing the entire surface of the pin to be working on the piston. The connecting rod bolts to the wrist pin with two bolts at a 90-degree angle. Bolting the rod to the pin forces the pin to articulate, helping in the oil distribution.
The piston design results in lower bearing pressures. Doers explains the importance of this design. “Two-stroke diesels are very high pressure and they don’t give this area a rest. With four-stroke engines, you are pushing your piston up and then gas pressure pushes it back down. And then you push the piston back up when you are pulling in fresh air. When you do that, you have negative work on the piston. So the piston will pull up and allow oil to get into the bearing area. But on a two-stroke, you push your piston up and gas pressure pushes it down. Exhaust rushes out and fresh air rushes in and it pushes it back up. You never get a rest. So you just don’t get oil into this area. So it is important that you design it very well. We have oiling holes coming from the top and we have oiling grooves so we get oil in there from within the piston. And the large area distributes the load, so we keep the psi loading on the bearing down.”
On the pistons, the crowns are stainless steel and the skirts are aluminum. The temperature of the diesel engine combustion process is around 4,500 degrees. If an all-aluminum piston were used, it would basically turn to slush with the high temperatures. So the stainless-steel crown protects the aluminum piston from the heat.
The cylinder heads are screwed in to the top of each cylinder. The cylinder heads are water-cooled and contain several components. The fuel injector is located on the center of the cylinder head. A stainless-steel fire deck is used for the same temperature resistance as the steel crowns that are used on the piston. In order to help hold the fire deck in place, a steel spring that looks like a large beveled washer is used. It is just outboard of the fire deck. When the cylinder head assembly is torqued down, the spring actually flattens about 0.035 inch. This applies about 25,000 pounds of force, preventing the high-pressure gas in the cylinder from escaping out the cylinder head.
The cylinder head is not torqued to a specific torque. Rather, it is torqued to a deflection value. The cylinder head is drawn down until it is aligned with an index mark. That ends up being equivalent to approximately 1,200 foot-pounds. With that much force required to torque the cylinder head, a 10-to-1 torque multiplier is used to torque it down. When using the torque multiplier, the other cylinder head is used as a reaction post for the torque multiplier, allowing one person to ratchet the head in.
The fuel system on the engine is a mechanical one. An engine-driven fuel pump delivers pressurized fuel to the injectors. Cams within the pump control timing. There are two cams on the camshaft. Each cam controls two cylinders. In order to enhance safety, there are four separate mechanical systems for fuel supply. Each cylinder has its own fuel supply system with injectors and fuel lines.
The fuel injectors are installed on the center of the cylinder head. The injector has flow straighteners, check valves, springs, and other components within the body of it. The pressure opens at 3,000 psi and fuel is delivered at about 20,000 psi. This allows for the fine atomization necessary for an efficient fuel burn.
The fuel injector is one of the few parts that mechanics will need to service on the engine on a regular basis. It will require regular checks at engine TBO. The check involves a tear down, tip replacement, flow check, and spray pattern check. Authorized service centers will perform the checks.
If an injector needs to be replaced for any reason, the process is fairly simple, much like changing a spark plug. You simply remove the fuel line and unbolt the injector. When you get the replacement injector, you simply take it out of its wrapper, put it in the engine, and bolt up the line. The fuel line then needs to be purged because if air is left in the line the engine will be difficult to start. To purge the line, you simply crack the nut until fuel is flowing out and then re-tighten the line.
The engine has an external oil pump. This will make any oil pump maintenance fairly easy to accomplish. Unlike an internal oil pump that requires engine disassembly to get to the pump, all that is required to remove the oil pump is to disconnect the lines and remove the four mount bolts.
The oil used for the engine is specifically formulated for diesel engines. Doers stresses the importance of using only approved oils. Regular oil is not compatible with a diesel engine. Additives for scavenging the lead out of the engines would cause damage to the diesel engine. Oil change intervals are expected to be at 50 hours to keep in line with existing practices on gasoline engines. The company is also looking at the possibility of using a synthetic oil in the engine, and engine oil change requirements could be extended if it does go with a synthetic oil.
There are currently three test vehicles being used to test the engines. An engine is installed on an aircraft, which has about 30 hours flying time in many different testing parameters. A trailer rig is also set up with another engine and a propeller to test components as they are developed. A third engine is mounted on a testing dynamometer.
This has been an introduction to the DeltaHawk diesel engine — an engine that could be making its way to your hangar in the near future. Diesel mechanics needed for an aircraft repair facility? This may not be a far-fetched idea after all. We will follow this exciting program and keep you informed on developments as they occur.