Rory Jackson reports on how the concept of a rotary exhaust valve maximises the merits of this supercharged UAV two-stroke
Source: Unmanned System Technology
The city of Fenton, Michigan, is home to Strange Development, an engineering company aptly named for its predisposition towards new and unconventional solutions for powering vehicles. Based not far from the ‘Motor City’ of Detroit, the company was founded by CEO John Krzeminski about 15 years ago as an r&d workshop focusing on the design, engineering and manufacturing of advanced vehicle power plants.
Krzeminski and his team’s foremost ambition was to identify and resolve the most pressing needs of the UAV market, seeing it as the best outlet for their passion for developing new engine architectures. And indeed, Strange Development has produced a new engine entitled the REVolution, which is designed primarily for UAVs but can also be customised for marine, road and other vehicle applications.
The REVolution is a supercharged two-stroke inline two-cylinder measuring roughly 400 x 350 x 300 mm, with a maximum output of 220 hp (164 kW) – which it achieves at 6800 rpm – and a BSFC of 221 g/kWh when performing at these levels. It had been trialled at up to 7000 rpm at the time of writing, with 8000 rpm as the expected redline.
It runs on gasoline, has a BMEP of 25 bar, and weighs 49.89 kg (110 lb) including the oil pump, supercharger and a front-end accessory transmission system that drives both of them. The standard-issue electric fuel pump and dry oil sump are external to the main design (although a pan for collecting oil via gravity is bolted under the crankcase). Thermal management is provided largely by an internal water-cooling system, and integral oiling channels provide further cooling, along with lubrication.
The REVolution’s origins
Between 2008 and 2010, Krzeminski and his team were performing some work on a four-cylinder, four-stroke, carburetted engine to be used on a UAV – which left Krzeminski somewhat nonplussed.
“What surprised me was how heavy it was – and still is – relative to its actual power output,” he says. “There are ways to improve it: it’s a well-known model, and engineers slap turbochargers and fuel injection systems onto it all the time, but the base architecture is heavy and outdated.
“We determined that a much higher power-to-weight ratio would be necessary to draw users away from that comfortable, suboptimal-but-reliable type of engine to a newer, more innovative one.”
That motivated Strange Development to start considering what such an engine would look like. Soon after, the team started working on software designs and CAD simulations for a poppet valve-controlled two-stroke. This design was inspired by Detroit Diesel’s uniflow-scavenged configuration (referred to in the Apple Tree Innovation dossier, UST 28, October/November 2019) but in late 2012 the team decided against this approach.
“With poppet valves, you’d get all the mechanical complexity, cost and weight of a four-stroke valvetrain, and Detroit Diesel’s engine was designed to run at maybe half the revs we were going to need,” Krzeminski says. “The valvetrain dynamics would be great on a diesel, but not for the gasoline engine speeds we were looking to tackle. It also just wasn’t going to get us the power-to-weight ratio we wanted, so we shelved that design temporarily and focused on our engineering services for a while.”
In 2014, the company was asked to provide r&d for a two-stroke that used turbocharging to compensate for altitude effects. This spurred an in-house discussion about forced induction, with a focus on how best to control the flow of gases through the two-stroke combustion chamber to boost power output.
“That four-stroke engine manufacturer I mentioned earlier, for example, will always put the turbo after the combustion chamber, because you’d want a measure of the wave dynamics to push the charge air back into the chamber,” Krzeminski says.
“And then the client asked us about supercharging their design. So we built a test engine, put it on the dyno, and witnessed it actually losing power, because it wasn’t getting any energy back from compressing the gas.
“Once the compressed gas was pushed into the engine, the piston would come down and the extra charge would just escape out through the exhaust. We drew all of this out on a whiteboard and laid it out for the client, and I remember pointing at their exhaust manifold and saying, ‘You need a valve, right here, to stop or slow the airflow from leaving the engine’.”
From these three observations – the need for high power-to-weight ratio, the excessive complexity of poppet valves, and the countervailing need for a valve to improve the scavenging and exhaust control to fully exploit the benefits of supercharging – the idea of the rotary exhaust valve (REV) was born.
The rotary exhaust valve
In 2015, Strange Development started running 1D simulations of a two-stroke with an axial-flow cylindrical rotary valve similar in principle to that developed by Bishop Innovation and Mercedes-Ilmor for the exhaust port in Formula One V10 engines. The company used the bottom half of its previous poppet-valve engine design but redesigned the top half to use a REV instead.
The team quickly noticed that not only could the exhaust valve increase the two-stroke’s power density, and yield more from supercharged induction, but depending on how the valve was timed, it could also enable better control of greenhouse gas emissions.
Considerable design work has gone into the timing of the rotary valve system and optimising it for the airflow in and out of each cylinder. As a result, the typical REV unit has a few key specifications.
The valve is machine-cut from stainless steel (to prevent damage or thermal expansion from the exhaust gases, which can reach temperatures of up to 1200 F (649 C), and its port measures about 68 mm in diameter. “We might make it from titanium to save weight in the future, or from aluminium with a thermally resistive coating,” Krzeminski adds.
“The way we cool the valve – with water circuits and oil injection – is absolutely critical to ensuring that it operates smoothly, as it fits with very close tolerances inside a chamber that runs across the two exhaust ports, and we can’t have it or its bearings expanding and sticking from the heat.
“As for sealing, we don’t look at the valve like a poppet valve, which has to provide 100% sealing; the valve is there to control flow dynamics, not to completely seal. Even so, whenever the piston crosses up past the exhaust port, sealing by the valve is no longer needed because the piston provides all the sealing necessary.”
So although each valve has two seals – one on each side of its cylindrical section – the valve is designed with the knowledge that it will experience some marginal level of leak. The company sees this as a valid trade-off; in exchange, the seals are subjected to a minimum of friction, which reduces their associated wear and parasitic losses, and allows the valve to spin freely.
The two valves rotate parallel with the crankshaft, with the timing factoryset by a belt drive on the engine’s front and a quadrangular push-fit interlock connecting the two valves as a shaft at 90º to each other.
The way in which this engine operates and scavenges is not to be confused with a conventional two-stroke’s crankcase compression.
The traditional two-stroke will forgo valving (aside from reeds or other systems on the throttle) because it relies on the piston as a pump to carry out internal pressure changes and dynamic fluid movements. Most often, the piston moves upwards (lowering crankcase pressure) to draw fuel and air into the crankcase, and when it thrusts downwards it pushes the fuel-air mixture out of the crankcase, through transfer ports up into the cylinder.
In the REVolution, however, the supercharger provides a persistent feed of high-pressure air directly into the cylinder, with a flow rate of 6188 litres/ minute at peak power, eliminating the need for the piston to act as a pump. Instead, the piston acts solely as a valve for the intake air, shutting off the feed from the supercharger at key moments.
With the exhaust port positioned slightly higher than the intake port, the piston leaves the exhaust open for combustion gases to escape when needed. When it needs to be closed off, to catch and compress the intake air, the REV shuts off this passage, as per Krzeminski’s earlier recommendation.
The added element of having a rotary valve in the exhaust port creates a ‘sixcycle, two-stroke’ operation, with six stages or events taking place during each revolution of the crankshaft: exhaust, cylinder flush, cylinder charging, injection, compression, and power.
These stages are defined by the relative positions of the piston and the REV, with the latter driven by a belt and gears on the front of the block (and initial start-up provided by a standard 12 V starter).
In the initial, exhaust, phase the piston thrusts from top dead centre (TDC), opening the exhaust port. At this stage the REV’s passage is lined up with the exhaust passage, allowing combustion gas to exit freely, as it would in a conventional two-stroke.
However, the difference becomes noticeable as the crank angle goes from about 90º to 120º, to begin cylinder flush. By the latter point, the piston crown is nearly at the bottom of the exhaust port, and the intake ports are partially opened, but the REV is continuing to revolve backwards, closing the exhaust port.
“Again, if intake air escapes out through the exhaust, it’s a waste of the supercharger, and reducing the oxygen content in the exhaust allows for add-on technologies such as catalytic converters that you don’t usually get to use with a two-stroke,” he says.
“With the REV changing the exhaust port ‘shape’ by closing upwards helps draw the incoming scavenged air in an arc towards the cylinder head, which helps a lot with fuel mixing later.”
As mentioned, the engine’s unconventional scavenging design does not feed air into the crankcase. The intake air path is completely separate from the lower crankcase and the oiling system, with air fed directly into the cylinder via an intake piston port, and the piston behaving as the valve mechanism. As the piston uncovers the port, air flows in and is stopped when the port is covered again.
So long as the intake port is uncovered, the low position of the piston ensures in-cylinder pressure is lower than the pressure of the supercharger air. The latter is typically around 1.2 bar, although this changes depending on where the engine is performing in its map and operating range, ensuring air flows undisrupted into the cylinder. In-cylinder pressure will not rise above intake pressure until after the intake port is already covered.
This brings the biggest benefit of Strange Development’s rotary valve – typically, when a piston is at bottom dead centre, a direct channel is opened between the intake and exhaust ports. With the company’s custom valve shutting this off to control the scavenging, internal cylinder pressure can be raised above atmospheric.
“This is where we get our power density from,” Krzeminski says. “A traditional twostroke cannot effectively supercharge without something stopping exhaust flow. Companies can try altitude compensation, pipe tuning or other approaches, but our method will actually significantly raise cylinder pressure above atmosphere in a reliable and consistent way, to an extent we’ve not seen in any comparable engine. Our long-term target for precombustion pressure is about 15 psi above atmosphere.”
Also, by having a supercharger and a valve for controlling it, the combustion pressure can be changed dynamically. This means that if any future version running on heavy fuel (or other fuel type prone to knock) began detecting knocking, the REV and supercharger speeds could be slowed down to limit its occurrence.
To achieve the optimal swirl of air for mixing with fuel in the injection stage, the cylinder liner has five scavenging air ports. Each is only half as tall as the exhaust port, so the intake stays cut off during exhaust. With the exhaust port, this gives a total of six ports.
“The liner actually looks a lot like a traditional two-stroke sleeve, but with a single larger exhaust port and without reed valve intake ports. We’re controlling upper cylinder pressure purely with the REV’s sealing,” Krzeminski notes.
“The most important scavenging air ports are the ones immediately next to the exhaust port. If we had only the crossflow-type ports that sit across from the exhaust, our CFD showed that the air would travel straight across the cylinder, with nowhere near enough scavenging of this air out of the chamber.”
The air from those Schnuerle-like ports ‘catches’ the air from the crossflow ports in the middle of the chamber during the charging phase, so it mixes just how we want it to in the injection phase.
“Balancing the flow from intake ports is something Detroit Diesel always had problems with in their uniflow-scavenging system. Turbulence can be a bad thing for uniflow systems, because you want almost linear movement of the air as a monolithic ‘slug’, but for us, turbulence is actually quite useful.”
Fuel is sprayed at 50 psi and an angle of 15º, after the REV is closed but before the intake port has shut, to avoid pushing fuel out with the exhaust.
The compression stage starts once the intake port is fully shut off. Krzeminski notes again that in most two-strokes the exhaust port would still be slightly open at this point, resulting in less pressure build-up.
Finally, the power phase starts as the piston nears TDC. The air inside the cylinder is compressed completely for ignition; outside the cylinder, although the exhaust port is now shut off by the piston, the REV has reopened almost fully. Once combustion occurs and the piston thrusts back down into the crankcase, the valve will again be wide open to enable the operating cycle to be repeated.
“The rotational momentum of the valve runs on fairly loose roller bearings, and receives ample lubrication. These keep losses and friction down to a minimum,” Krzeminski says.
After successful 1D and 3D simulations of the new engine and valve concept, Strange Development began prototyping the system to begin testing and iterating its designs. Through its in-house cutting and casting machinery, the team was able to make quick adjustments to geometries and tolerances wherever it wished.
“We probably went through 10 iterations of the cylinder head geometry alone, for optimising the airflow and mixing dynamics inside the top of the cylinders,” Krzeminski says.
“We’ve been running it on the dynamometer for six or seven months, and have accumulated nearly 400 hours of testing. We built a 300 hp dyno and customised pretty much everything on it, other than the head itself, to suit the engine’s specialised design.”
Most of its components have been machine-cut from 7075 aluminium alloy. This was selected for having the highest strength-to-weight ratio among the available metals, and for its track record in aircraft engineering.
“And because it’s so strong, we’re confident we can reduce our weight below 110 lb,” Krzeminski says. “We’re engineering on the safe side for now but we also have side projects aimed at weight optimisation, since that’s crucial for UAVs.
“Complexity and time between overhauls [TBOs] are big bottlenecks for two-stroke performance as well. Many two-strokes have lubrication problems that mean their top ends have to be torn down every 50 hours or so, replacing pistons and rings and so on, which of course is time spent not flying.
“So we’re aiming for a 1000-hour TBO on the REVolution, with just a minor oil change every 50 hours, and the engine can be stripped and rebuilt from scratch in less than 4 hours.”
While the engine is oil-injected in a similar manner to a four-stroke, the company wanted to be able to use as little oil as possible, and eliminate even the marginal oil consumption levels that four-strokes are prone to.
As discussed, the REV forms one key part of the cylinder airflow management. The piston forms the other, and has thus received almost as much attention.
The crown is coated using a ceramic, to protect its exhaust side from being damaged by the high-velocity, superheated gases that rush out of the exhaust port as it opens. Krzeminski notes such damage is a perennial issue in the two-strokes he has observed over the years, and that Strange Development is also considering coating the valve in a ceramic to provide similar protection.
The piston skirts are coated in molybdenum disulphide (or ‘moly’) for long-term durability and low friction during their movements against the cylinder walls, as well as being able to survive the extreme heat of the cylinder.
Each piston pin has three trapezoidal piston rings, a shape that allows enough room for the rings to float stably in their grooves. The bottom ring sits at the end of the moly coating, preventing intake and exhaust gases from entering the crankcase, and keeping the crankcase’s oil out of the cylinder’s airstream.
“We want to stop excess oil flow from touching the moly, because that would contribute to emissions, smoke output and other problems,” Krzeminski explains.
The crankshaft separates into three sections at the two crank pins. In addition to press-fitting the shaft pieces together, customised holding pins are routed through the crank throws and pins to stop rotational differentials occurring between the sections, otherwise they might start to spin at different speeds near maximum power output.
Each con rod sits between two counterweights, and in the centre of the crank is a gear for driving the water pump, which sits at the bottom of the crankcase, perpendicular to the crankshaft. That propels water up through the cylinder jackets, above and below the rotary valve seats, and up to the cylinder heads.
On either side of the water pump gear is a seal for separating the undersides of the cylinders. Keeping them airtight is key for preventing losses of charge air throughout the crankcase.
The crankshaft interfaces with the engine block via six steel ball roller bearings – one at the front, one at the back, and two on either side of each counterweight. Four more such bearings are installed on the two REVs, and four needle bearings are installed across the two con rods, for a total of 14 internal bearings.
The ECU controls the engine using a speed density strategy, mapping intake charge air pressure and temperature against engine rpm to determine fuel injection and ignition adjustments across varying speeds and altitudes.
Also, since airflow can be controlled via the exhaust valve end and the throttle, there is potential to layer a throttle control strategy atop the speed density to increase the accuracy and speed of adjustments in the REVolution’s performance.
To further assist in isolating the cylinder gases, six reed valves (arranged in two rows of three) sit under the crankshaft. Unlike with most two-stroke reed valves though, these six are not used for air intake.
Instead, as the piston comes down, the valves provide an evacuation route for pushing the air beneath the piston out of the crankcase. The force of this air also helps push oil collected in the oil pan back out to the external dry sump.
“We don’t use a scavenging component on the pump, as you might in a traditional dry-sump arrangement where you mechanically pump out the oil; we’re just using the engine as a pump,” Krzeminski says.
“When the piston goes back up, the reed valves shut. That effectively creates a vacuum under that piston, limiting the rotational losses, because you’re not forcing the con rods and crank parts to move and spin through air anymore.
“If you have a diesel engine running at 1000-3000 rpm, that’s not really an issue, but with a two-stroke gasoline engine spinning at 7000-8000 rpm, there are actually a lot of losses owing to air-induced drag inside the crankcase. That’s why our engine is designed the way it is, with the reed valves and shaftmounted seals.
“It’s not a totally new idea. For example, there have been a few studies in fourstroke designs that used reed valves in this way to reduce drag losses, but our system was designed from a blank sheet to suit our requirements for vacuuming the air and pumping out the oil.”
Each cylinder has a factory-set injector bore angle as well as a perpendicular cylindrical valve seat. The cylinder liner is iron alloy (with carbon chrome and molybdenum) and as discussed it has five air ports and one exhaust port designed to optimise the scavenging dynamics.
The fuel injectors sit inside the intake port, very close to the piston clearances. “We’re using two extended-tip injectors per cylinder,” Krzeminski says, “There’s actually very little space between the fuel injectors and the piston skirt, and immediately after cylinder flush the injectors spray right after the piston thrusts down past them, almost directly into the cylinder.
“While this is technically port injection, because the injectors sit in the intake port and spray from inside the port, it actually works almost like a low-cost version of direct injection. There’s effectively no fuel in our intake airstream.”
Four injectors are installed because earlier CFD studies showed that using two points of injection per cylinder with smaller injectors provide more balanced atomisation and fuel-air mixing at wideopen throttle (WOT) than a single larger injector. This arrangement also allows one injector to be switched off when idling, to reduce emissions.
“Having trialled this approach for a while, we’re increasingly limiting the use of the second injector to WOT, to increase the accuracy of fuel usage.”
Strange Development has also trialled direct injection systems in the earliest versions of the REVolution, in which the fuel can be injected any time before a spark and after the cylinder flush stage.
Krzeminski explains, “We went to port injection for the standard version to reduce the cost and complexity of the prototype. We can easily add more complex subsystems once customers are used to running and maintaining the basic REVolution, and they’re confident in our ability to provide quick servicing and replacement parts when needed.”
The cylinder head is made from two aluminium plates, with 12, M8 fasteners – six around each cylinder – bolting the upper plate atop the lower (and both onto the engine block).
Once fastened together, the head encloses a figure-eight-shaped cooling jacket designed for balancing water flow and thus temperature as needed across the tops of the two combustion chambers; a pair of inlet holes allows the water to enter upwards from the cylinder jackets. The upper plate also has an exit port for the water to leave the engine through an external hose.
The assembly of gears and belts on the engine’s front is removable and modular, in order to accommodate any need for a UAV to drive different alternators or package the ancillary systems differently.
“Modularity for hybridisation is key for reducing operating costs, heat signature and noise, without relying on the 30 kg of batteries you’d need to achieve the kind of endurance a fuelled range extender can give,” Krzeminski says.
“I worked for a time for two major US automotive OEMs, and I noticed that their intake manifolds, exhaust arrangements and accessory drives were always different depending on the engine packaging they were paired with. We took that kind of approach, to enable quick modifications of everything around our engine block, especially when seeing how many UAVs were using engines built specifically to fit within their airframes.”
He adds that his automotive experience proved very useful when designing the engine’s transmission, resulting in something akin to a serpentine belt system for distributing the crank force to other onboard processes.
The transmission assembly has two timing belts. An upper belt looks after the timing and control of the valves and oil pump, while a lower belt drives the supercharger.
In the upper belt, the crank force is delivered via an idler wheel that sits above the crankcase on a bearing mounted on the transmission assembly’s inner front plate, and is directly actuated by a crank gear near the front of the crankshaft. The belt runs over the top of the idler, and is kept taut by running down beneath the valve gear and the oil pump gear.
The forwardmost of the two REVs is machine-cut, and has a shaft extending out of the valve seat chamber, with a gear fastened on its end.
A larger crank gear sits on the very front of the crankshaft, driving a sprocket on the front of the supercharger via the second timing belt. A tensioner wheel presses down on the upper part of this belt to secure it.
“We’ve had to take care of a lot of different belt dynamics, and we’ve put a huge spread of design choices into the transmission to ensure the right tension and support during long-run operations,” Krzeminski says.
Although the engine previously used a centrifugal-style supercharger, it was originally designed with – and has returned to using – a Roots-style supercharger (with a screw-type Roots charger from Eaton expected to be used in full production). It supplies 500 cc of air with each revolution of the crankshaft.
“When we used a centrifugal charger, we had a lot of idle-stability problems, because we simply weren’t moving enough air at idle,” Krzeminski recalls. “We looked at several types of supercharger, and simulated and dynotested a range of them. The centrifugal charger works with no problems at higher rpm, but the idle stability was so poor that we switched back to Roots-type positive displacement.”
Strange Development plans to develop a twin-charged REVolution. This will use a high-efficiency Roots supercharger to help with idle and low-rpm stability without incurring parasitic losses, and a turbocharger at the higher rpm – most likely a BorgWarner model or one from Garrett Motion – to better drive the airflow through the engine. It also intends to use an intercooler between them to cool the charge air, resulting in what the company envisions as an even more power-dense version of the engine.
As mentioned, the company is also working on a heavy-fuel version of the engine, which will feature moderate changes to the cylinder head and injection system, although the block and other components are expected to remain largely unchanged.
At the time of writing, Krzeminski and his team were finishing calibration work on the standard REVolution, studying how the engine responds at different throttle inputs. The next planned step is to run UAV-specific durability cycles before planning flight tests with an appropriate partner.