Machine Design

Rethinking engines

Rocket engines and the internal-combustion cycle get a remake, thanks to engineering ingenuity.

Senior Editor

A four- cylinder split-cycle Scuderi engine should be as powerful as a traditional six-cylinder engine. It should also be able to burn almost any combustible fuel.

A model of the IPD shows the intricate plumbing needed to build a full-flow, staged combustion rocket engine. The throttleable engine can be shut down and started in space. The technology developed should see use in NASA's planned return to the Moon and follow-on flights to Mars.

The oxygen preburner built by Aerojet generates oxygen-rich stem that powers the oxygen turbopump. About 8,000 gpm of liquid oxygen will flow through it on its way to the main combustion chamber.

Nighttime was the right time to test-fire the IPD at NASA's Stennis Space Center in Bay St. Louis, Miss. Hurricane Katrina damaged the facility, which set back the test schedule, but the program was successfully completed.

When it comes to engines, smaller and more powerful are always better. Throw in less expensive and you've hit the powerplant trifecta.

It seems two design teams have managed to do just that, one with the traditional internal-combustion and the other with rocket engines.

The internal-combustion engine (ICE) with its four-stroke Otto cycle has been around for more than a century. Engineers have managed to coax more power and efficiency from it over the years, but they're starting to come up against technical obstacles that can't be overcome without as-yet-undiscovered materials or overly complicated designs.

Fortunately, Carmelo Scuderi, a mechanical engineer with expertise in thermodynamics and fluid dynamics, came up with a new way to configure the ICE that would be more efficient and pack more power into a smaller package. And though he died two years ago, his family-run Scuderi Group continues to develop the technology and develop a working prototype.

Basically, the Scuderi engine splits the four strokes — intake, compression, power, exhaust —among paired cylinders. (Hence the term, split-cycle engine.) The compression cylinder pulls in and compresses air, then pushes it out through a crossover tube into the power cylinder. Carefully controlling the timing of the fast-acting poppet-style disc valves and pistons keeps air in the tube at a constant temperature and pressure, with one charge of gas leaving as another enters. A check valve in the tube could be used to ensure high-pressure air doesn't feed back into the compression cylinder.

In the power cylinder, fuel is injected, the fuel-air mixture exploded, and power extracted. The power cylinder also sends exhaust gases to the atmosphere. With both pistons connected to the same crankshaft, there is one power stroke per crankshaft revolution, compared to one power stroke for every two revolutions in traditional ICEs. The cylinders can be offset from the crankshaft at almost any angle to increase efficiency or to make a smaller overall package.

This split-cycle engine has several advantages. For example, each cylinder can be tailored in terms of bore, stroke, connecting rod length, and offset for specific tasks. The compression cylinder, for example, can be sized to provide a wide range of compression ratios. This lets designers essentially supercharge a Scuderi engine (i.e., ensure the power cylinder burns a mix of highly dense air and fuel) without any additional engine components. And because there is no combustion in the compression cylinder, it can be lighter and made with less material.

"We also don't need extra space for a combustion chamber above the piston in the compression cylinder, so we can get extremely tight clearances, less than a millimeter, between the piston and cylinder head at the top of the compression cycle," says Salvatore Scuderi, president of the Scuderi Group. "That can give us compression ratios as high as 100:1. We actually compress air to postcombustion levels."

The compression cylinder can also be designed to produce more compressed air than the power cylinder can use. Excess air is at the heart of the company's plans for what they somewhat mistakenly call their hybrid engine. Unneeded air is piped from the crossover tube into a holding tank where pressures can reach 5,000 psi, according to Scuderi. If that tank is large enough, the engine will run on stored air for some time, giving the engine a boost in efficiency and power as it shuts down one or more compression cylinders. The tank can also be charged before the engine starts, giving the operator the option to use stored air for quick boosts or to run totally on air for a short time.

And unlike current engines that close off or off-load cylinders when their power is not needed, the Scuderi engine leaves the intake valve open, but shuts the valve to the crossover valve. "So the engine still pulls in air and compresses it, doing work, but the compressed air in the cylinder acts as a spring to bounce the piston back and recover some of that work. And low pressures make frictional losses small," says Scuderi. "This is more efficient than pulling in air then pumping it out, as is done in other engines to off-load cylinders."

On the power-cylinder side, engineers can take advantage of the Miller Effect by making the power cylinder longer. This lets the engine extract more energy from combustion as it lets the combustion gases expand as much as possible.

Another important factor in the design is getting the power cylinder to fire immediately after the piston passes top dead-center position. The inrush of compressed air from the crossover tube and the action of the power cylinder just starting to move down creates tremendous turbulence in the fuel-air mix in the piston. "This causes an extremely fast burn," says Scuderi. "And a faster burn rate improves engine efficiency."

Burn rates are usually measured by the degrees of crankshaft rotation needed to complete combustion. The fastest burns in conventional engines are on the order of 22 to 24° crank-angle rotation. Scuderi says computer models of his engine predict burn rates approaching 10°. Fast burns let the engine work more efficiently despite firing after top dead center.

If a standard engine were to fire past dead center, combustion would not build pressure fast enough with the piston always traveling away from the explosion. So the cylinder must fire before its piston reaches top dead center. Hence the piston races into the hottest part of the flame, increasing peak temperatures and wasting energy trying to recompress combustion gases.

In contrast, pistons in the Scuderi's power cylinder rush away from the flames. So even though average temperatures are higher in the power cylinder than in a conventional cylinder, peak temperatures are lower.

And lowering peak temperatures reduces NOx formation by up to 80%, according to simulations. Combustion is also more complete and less fuel is burned in the Scuderi. This cuts CO2 and hydrocarbon emissions.

Altogether, simulations indicate the Scuderi design should boost engine efficiency from 32% for a standard engine to over 42%. This translates into a car engine that delivers 30% better mileage. The engine cranks out higher torque at low rpms, which means more power at lower engine speeds. And lower engines speeds, with one combustion cycle power per crankshaft revolution, will reduce friction, along with wear and tear.

The Scuderi engine is not all that different from a conventional engine when it comes to manufacturing, says Scuderi. So making the switch to the new engine would require no major tooling changes and finished engines would cost about the same as conventional engines.

The design implications for diesels is even more impressive in that it reduces or eliminates three of the most expensive subsystems: injectors, turbochargers, and exhaust treatment.

Scuderi's built-in supercharging eliminates the need for add-on turbochargers. Combustion in only half the cylinders means only half the number of fuel injectors. Today's high-pressure injectors can account for a third of the base engine cost. And since compressed air entering the Scuderi power cylinder generates enough turbulence to atomize the fuel-air mixture, lower-cost low-pressure injectors can be used. Finally, firing after dead center significantly cuts NOx and soot emission, so large diesels won't need catalytic converters that can cost up to $50,000 and add another layer of complexity and maintenance. All told, Scuderi says his technology could cut the price of a diesel by 40%.

Currently, the Scuderi Group continues to develop the technology and is working on a prototype. They plan to license the engine and believe general-aviation aircraft are a good market. "Aircraft designers always need lightweight, powerful engines. And with its built-in supercharging and power density, the Split-Cycle engine should be a good match,' says Scuderi. "We could design a carbon-fiber tank to hold compressed air that would fit inside the wing. That way, the new engine wouldn't necessitate major changes in the basic design of wings or airframe."

But the Scuderi Group believes the engine offers benefits to companies that make engines for boats, cars, trains, off-highway machines, as well as the people who use them.

"There are still engineering issues, but there shouldn't be a showstopper," says Scuderi. He plans to field a working prototype later this year.

After 35 years without a major redesign, the liquid-fueled rocket engine is getting a facelift, courtesy of a team of engineers from NASA, the Air Force, Pratt & Whitney Rocketdyne, and Aerojet. They recently completed work on the Integrate Powerhead Demonstration (IPD), a full-flow, staged-combustion rocket engine that uses liquid hydrogen as fuel and liquid oxygen as the oxidizer. And according to IPD project managers, the engine met its design goals.

Compared to current rocket engines, the IPD demonstrated better fuel efficiency, higher thrust-to weight ratios, more reliability, and 250,000 lb of thrust, about twice the thrust of the Space Shuttle main rocket engines. Though only a prototype, one that will never take a payload from launch pad to outer space, the engine encompasses technologies and techniques that will likely see action as part of NASA's return to the Moon and plans for Mars over the next 15 years.

Like past rocket engines, the IPD uses staged combustion. This meant that a small percentage of the fuel leaving the turbopump went through the preburner where it was partially burned (or preburned in NASA parlance). This creates a stream of high-pressure exhaust gases that drive the turbopumps feeding fuel and oxidizer into the main combustion chamber. (An auxiliary system kick starts the turbopumps, getting them spinning before preburned exhaust is available.) In older engines, such as the Atlas, preburned gases are vented overboard once they've been used to spin the turbopump.

"As we evolved rocket-engine technology, we took those exhaust gases and routed them back into the main combustion chamber where they were burned along with propellant and oxidizer to generate more thrust," says Fernando Vivro, IPD program manager with Pratt & Whitney Rocketdyne. "That gave us better performance and more efficiency by more fully using all the fuel."

Earlier rockets also relied on a small stream of fuel to drive pumps for both the propellant and oxidizer. This made seals crucial in turbopumps that fed oxidizer to the combustion chamber. A small leak would put fuel and oxidizer in contact, potentially turning the entire rocket ship into a Roman candle.

To avoid this and simplify construction, IPD uses preburned liquid oxygen to power the oxygen turbopump and preburned hydrogen to power the hydrogen turbopump. The pumps, which both put out about 8,000 gpm, differ in that engineers had to design a multi-impeller turbine for the hydrogen side to ratchet up the pressure from inlet to discharge. The oxygen pump can get away with a single-stage design.

Roller bearings in both pumps were also replaced with hydrostatic versions. They ride on a cushion of propellant or oxidizer to reduce friction and wear, and cut down on moving parts. This should boost reliability and reduce maintenance.

The biggest change, however, is that all of the hydrogen and oxygen in the IPD are routed through their respective preburners, where only a small fraction is actually consumed. (A small amount of hydrogen gets sent to the oxygen preburner and a similarly small amount of oxygen gets sent to the hydrogen preburner to complete the triangle needed for combustion: fuel, oxygen, and heat.)

So in the closed-loop IPD, an auxiliary system starts the turbopumps, which send practically all the fuel and oxidizer through their respective preburners. The preburners consume a small percentage of fuel or oxidizer to make high-pressure exhaust, which powers the turbopumps and then goes to the main combustion chamber. Therefore, nearly all the hydrogen and oxygen go through the turbopumps and preburners and is then burned for thrust.

Sending all the hydrogen (or oxygen) through a preburner and a turbopump increases the mass of material sent through the turbopumps, compared to other rocket engines. This means the temperature of that material, in this case oxygen-rich steam and hydrogen-rich steam, can be lower, which should reduce maintenance headaches.

"We're lowering temperatures by about 400°F and replacing them with more mass flow," says Vivro. "And we still get higher pressures in the gases coming out of the hydrogen and oxidizer preburners. And high pressures and more mass in the combustion chamber translates into more thrust.

"As far as maintenance, we wanted to go from a complete rebuild of the engine between uses to more of a ‘wash the windshields, check the oil, kick the tires' kind of approach," says Vivro.

Aerojet Inc. redesigned the preburners to handle the increase in flow. They also had to come up with materials that could handle the fuel and oxygen-rich environments, as well as the temperatures and pressures. (What they came up with is proprietary.)

The design and materials let the IPD get away with a smaller combustion chamber. "The S-27, a rocket engine with 200,000 lb of thrust has a combustion chamber about 12-ft tall. Ours is three-fourths of that."

The chamber also boasts a channel-wall design, thanks to engineers at Aerojet. Traditional chambers use tubes braced or furnace braced into a bell-shaped nozzle. Fuel travels through the tubes to cool the nozzle and precondition or preheat the fuel prior to combustion. Chamber wall technology builds the nozzle out of an inner and outer shell. The inner shell has grooves machines on its outer surface. So when brazed to the outer shell, it forms passages for fuel to travel down and back, getting preheated as it cools.

The IPD should last for 200 missions, with overhauls needed only every 100 missions.

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