Could You Run an Internal Combustion Engine In Space?
It's not as dumb of a question as you might think, so we called in the experts.
https://www.motortrend.com/features/can ... -in-space/
TLDR: It's technical possible but it's not logistically feasible.Back when we posted about GM's efforts to send a vehicle back to the moon utilizing some of the same electric powertrain technology as the Hummer EV, a user on our Facebook page stated that this made sense because "an engine wouldn't work in space." While it would be easy to ignore such a comment, we decided it would be more fun to explore whether it is, in fact, impossible for an internal combustion engine (or ICE) to work in space. Just to make sure we weren't talking out of our butts, we also posed the question to researchers and professors at the California Polytechnic State University, or Cal Poly for short.
What Do Engines Even Do?
It's best to start with a refresher or potentially a new lesson for those who don't know just what an engine does and how it does it. The simplest idea is that an engine creates its own power by way of a "working fluid" to create motion. A working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. We know, it's hard to think of a gas as a "fluid," but in the world of science, gas is treated like a liquid for many things. If it didn't, we wouldn't understand how aerodynamics nor pneumatic valves work let alone be able to model them.
That definition is why an engine is different from a motor, which requires power from an external source to create motion, ie: an electric motor requires power from a battery or other electrical power source to produce motion. However, the layman may use either "motor" or "engine" interchangeably when talking about vehicle propulsion. For the purposes of this article, we will only use "engine" to describe an engine and we will not interchange the two.
The "C" in ICE
Combustion is the process by which a fuel is burned with an oxidizer at a certain ratio of each. This combustion creates the heat that causes expansion of the gasses in our cylinder—aka our working fluid. That's all combustion is and this is why most people, when asked, will explain why an engine will not work in space.
There isn't any oxygen in space and it's a vacuum that would suck out the fuel before it could combust, so it shouldn't work. What they don't ask themselves just before answering that question is "why does a rocket work in space when an ICE couldn't?" Now that you have probably asked that yourself, we can properly begin this article.
Suck, Squish, Bang, Blow in Space!
We reached out to Cal Poly to help us with this thought experiment on ICE in Space, and Professor of Mechanical Engineering, Patrick Lemieux, Ph.D., P.E.; and Professor of Aerospace Engineering, Dianne J DeTurris, PhD, were both happy to help us out and nail all of the theory and explanation down for this topic. We were happy to have their help—the two study and eat mechanical and aerospace engineering for fun and as a career.
The short answer is that it is possible to run an ICE in space despite the cold (to a degree—no pun intended) and vacuum of the environment. When it comes to how combustion works, everything is the same for gasoline and liquid rockets, it's just the amount of each liquid you need to achieve that combustion along with an oxidizer and the ignition event to get everything started.
For most liquid fueled rocket engines, ignition is created using a torch ignitor, but other oxygen-mixed propellants used hypergolic (self-igniting) fuels pumped into their combustion chambers, spark plugs (yes, like the ones in your car), or—in the case of the Soyuz rocket—"overgrown matchsticks" made from pyrotechnic flares that were mounted on birch poles. Once ignited, the fuel will combust and expand and propel the object the engine is strapped to. The internal combustion engine does the same thing, except those expanding gasses force a piston down to create rotational energy at the crankshaft.
Rocket, ICE, the Bang is the Same
"The difference comes in what you do with the energy produced," says Prof. DeTurris, "The rocket uses the energy to create thrust in a converging, diverging nozzle but the ICE uses the energy to create rotation. Either of these things can be done in a vacuum," however, she points out, "you just have to consider the surrounding temperature when designing your application and that could easily affect the materials you use in space." One such issue is due to the lack of oxygen, it is easy to cold-weld metals together. This vacuum related phenomenon allows metals to weld together without fusion nor heat, which has been a problem in the past for astronauts and satellites. However, modern materials and a better understanding of this phenomenon have led us to materials better suited to space and preventing cold-welding.
"You can get a 'feel' on how this affects things too," says Prof. Lemieux, "by considering the engines of small, general aviation propeller airplanes." "The normally aspirated ones see a dramatic drop in ambient pressure as they climb steadily, of course, and that's associated with a drop in performance and why 'density altitude' is such an important parameter for both engines and airplanes." That's why these engines are limited in altitude without the addition of a turbocharger or supercharger to force in more air, just as you would in a high-horsepower automotive engine. Boost pressure means more air to use while burning your gasoline.
Prof. Lemieux also explains that while it might look like the engine wouldn't work at all in a full vacuum, it is possible if you are able to supply an oxidizer. "Then that's certainly the case. If you relied on the surroundings to provide the oxidizer, it wouldn't work," he adds. If you designed the oxidizer injector to work with an enclosed plenum, you could even retain the same valvetrain designs we use on engines now. Or you could be innovative and remove the entire intake system and port, replacing it with a direct liquid oxygen injector.
Feeding the Powerful Rocket Engine
Using an oxidizer injector is similar to how liquid rockets do it now, it's just that an injector for a rocket doesn't typically work like the injectors in an ICE. The pumps for the liquid oxygen and liquid fuel of a rocket work a lot like a turbocharger and are called turbopumps. The difference—usually—is that instead of using exhaust gas to drive the turbine, it uses gravity and it pulls the liquid fuels down to drive a turbine. The impeller, attached to that turbine, pressurizes each liquid before sending it into the rocket's main combustion chamber.
There are others that use a gas generator to drive the impeller (working just like a turbocharger) and, recently, there have been efforts to drive the turbine with an electric motor (the "electric rocket" you might have heard of, provided you casually scour rocketry advances). How this is done simply differs by the rocket manufacturer and even the parameters of the particular mission the rocket is flying.
The pressurized fuel is fed into a main valve that opens and closes, controlling the fuel flow to the injector. What is actually atomizing the fuel is an plate (or pairs or sets of plates) full of precisely bored holes like you'd see on the end of a gasoline fuel injector. Except, unlike a fuel injector for your vehicle, there is no pintle that actually controls how much fuel enters the main combustion chamber. It's all controlled by the main valves, which control flow rather than volume.
Finally, the fuel is ignited, as we mentioned before, and the rocket lifts off from the pad or moves itself forward in space. To keep the fuel fed in gravity turbopumps while in space, without any type of separate mechanical or electrical pump, a rocket relies on the momentum created by accelerating to keep the liquid fuel and oxidizer flowing. This momentum creates a sort of artificial gravity that forces the liquids to the bottom of the tanks and constantly feeds the turbopumps. Many of these solutions to feed a rocket engine its fuel and oxidizer can be applied to an ICE. Again, it's just a matter of difference in what each engine is doing with the expanding gases.
The Vacuum isn't The Problem, Either
While you'd think the vacuum of space would pose an issue, Prof. Lemieux explains that the piston rings can seal in a vacuum. Keep in mind that those rings are fighting against the huge pressure difference of an expanding gas against the atmospheric pressure the engine would normally see. "What the piston rings are sealing against is not strictly the absolute backpressure in the crankcase," Prof. Lemieux explains, "Rather, it is the 'delta P' between the combustion chamber (CC) and the crankcase, which acts to push the content of the CC towards the crankcase."
He also points out that even when the engine is running at sea level, "there is a large delta P across those rings, which changes continuously throughout the 4-stroke cycle" and they do a great job of sealing the chamber throughout the cycle. "If the same engine is turbo (or) supercharged," he adds, "the delta P can increase significantly (say, more than 15psi), and the rings continue to do a good job of sealing it. Absolute 0 psi in the crankcase, which is your scenario, adds no more than 15psi to that delta P. So there is no problem there."
The Ultimate Way to Fight Backpressure
That vacuum environment could potentially be a benefit for an ICE. "On the mechanical side," says Prof. Lemieux, "things get interesting too: the lack of backpressure in the exhaust means that the engine volumetric efficiency would increase, so the engine performance such as Brake Mean Effective Pressure (BMEP) and others would go up." That also works within the crankcase, which he notes "would also drop, and that means that the pressure differential across the piston face would go up by up to one atmosphere, bumping BMEP up yet again." If you saw Engine Masters season one, episode nine, you know that engines of all types want reduced backpressure, and that there is power to be gained by reducing it. Just imagine the power your engine could make with zero backpressure in the exhaust or the crankcase.
All of that is to say that vacuum isn't an issue and that combustion doesn't really rely on the "compression." It's really more of a storage of rotational energy that gets transferred to the transmission by way of the crankshaft. However, that compression does result in heat as the gases compress, and that along with the spark from the spark plug, begins the conversion of gasoline and oxygen into a thermal expansion of those gases.
So, What Does the Compression Stroke Really Do?
However, if you can generate enough heat from your spark, or even use a pre-ignitor, your combustion chamber doesn't need compression and would continue to work. There have even been tests with engines that use a separate combustion chamber that feeds the expanding gases into the cylinder to force the piston down. Again, a rocket engine does the same thing and doesn't have a piston to create compression. You can also ignite gasoline outside an engine, it's especially easy when it is in its gaseous state (the fumes).
The compression of any piston engine is a way to store potential energy that will be used to generate rotational energy by way of the crankshaft. Doesn't matter if it's two- or four-stroke; gasoline, diesel, or any other form of fuel. If fuel is hot enough to reach ignition with its oxidizer, it will ignite and expand until it hits something and moves that object or stops because that object requires more force than that expansion is producing.
It's Not the Fuel and Oxidizer, It's the Weight
Other than the extreme cold, which could be accounted for by materials right now (parts in space need to rotate, too), why don't we see ICE-powered generators for the space station, the Perseverance Mars rover, and future moon buggies? There are two important considerations when it comes to space exploration: weight and longevity. Sure, we have the ability to inject liquids into combustion chambers, though oxygen is a cryogenic liquid and requires very cold temperatures to stay a liquid, nor is that an issue as we're able to do that in rocket engines just as we do with many types of fuel.
The problems arise in carrying that fuel and oxidizer into space and how you would potentially replenish it. One of the major problems with getting into space is that you need a lot of velocity to get into orbit and even more when you're looking to get outside of Earth's gravitational influence and on to another planet. This is why you see many orbit and interplanetary missions use stuff made of things race car builders dream of such as titanium, carbon fiber, and other superlight materials.
It's also why so many space and martian-landing vehicles look like they have been skeletonized save for some of the foil shields to protect thermal-sensitive parts. If you also need to carry the fuel and oxidizer, you've got to account for that mass in your launch and orbital mechanics by applying more thrust energy to achieve the escape velocity. If you've dealt with a race car, you see where this is going. If not, more thrust requires more power and that means more fuel and more weight. If you could refuel in orbit—which at the time of this writing, we couldn't—this wouldn't be an issue. Since we can't, we rely on batteries that are fed by solar power to drive motors and power electronics on our space vehicles and the International Space Station (ISS).
A Side Note on the Perseverance Mars Rover
We are as yet unaware of any resources that would allow us to replenish our fuel or oxidizer on the Moon or Mars. This is where the Curiosity and Perseverance differ from other Mars missions, instead of relying on just solar panels to power their batteries, these sedan-sized rovers use a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), essentially a miniaturized nuclear power plant.
The main difference between your local nuclear plant and the MMRTG—besides the obvious size difference—is that instead of turning water into steam that turns an electric generator turbine, it uses the Seebeck effect. The simplest way to describe the Seebeck is that two dissimilar, but electrically conductive materials create electricity by applying a difference in temperature at each end of those materials. Essentially, it's the reverse of a Peltier device used in seat coolers, where an electric current is passed between those two materials and creates a temperature difference in the two materials, one side hotter and the other colder; it is how many vehicle refrigerators work without needing freon and a compressor. All said, we're not going to see an ICE-powered Mars rover or even a Moon buggy anytime soon.
Sparks in Places Where Astronauts Live
There are those of you who point out that fire in space equals bad, probably recalling Apollo One and the loss of Gus Grissom, Ed White, and Roger Chaffee while still on the launch pad for a launch rehearsal. You've also, no doubt, heard about the warnings of having a flame near pure oxygen and pictures of burned rooms and worse. But of course oxygen, absent any fuel source, poses no fire risk. It is true, however, that any fuel will burn more intensely in an atmosphere of pure oxygen than it does in air. That's because the nitrogen that comprises roughly 80 percent of the air we breathe is not an oxidant.
In today's spacecraft and future space stations, the atmosphere is equal to what we have here on Earth: 20 percent oxygen, 80 percent nitrogen. Translated, that means the fire risk on the ISS is equal to what it would be here on Earth, just very, very far away from the nearest fire station.
What About The Byproduct of Burning Fuel?
If we were to retain the use of gasoline and oxygen as an internally fitted ICE's fuel source, then exhaust would be a problem in an isolated environment. The carbon-dioxide, nitrogen-oxide, unburned hydrocarbons, and other particulate matter would need to be filtered to create a safe environment in which a human could work within. It'd be ideal if all of those gases could just be released in space, but that would be a complex endeavor, meaning the more realistic use case of an ICE-powered generator would be one that is exposed to the environment of space, just as a rocket engine is.
If we used an alternative fuel, then their byproducts would also need to be filtered. If we used liquid hydrogen, for example, the resulting byproduct would be water with traces of hydrogen peroxide and ozone, making it still not great to ingest straight from its tailpipe, but better than gasoline. An ICE would need the same protections from the thermal swings of passing between the sun and the planet, but those could be easily solved with heaters and thermal covers.
We Could Run an ICE in Space, But…
While it is possible to run an internal combustion engine in the vacuum and cold environment of space, the reality is that it's just not feasible. The weight of carrying the fuel and oxidant is the main burden, followed by the challenge of replenishing both when off Earth. This means that battery, solar, and nuclear power sources and generation are the only reliable and sustainable sources for space stations and vehicles that need power for instruments and even motion.
Outside of rockets, we'll never see an ICE-powered planet exploring vehicle. Consider your dreams of a sweet gas-fed moon buggy crushed into the ice crystals your tears would make in the cold of space, at least until those vaporize away when they meet the light of the Sun. Bleak, right? That said, this hardly means a space vehicle needs to be boring. Imagine a 1,000-hp, all-wheel-drive moon buggy with wild steering based on the Hummer EV's Ultium technology. That would certainly be doable. Wonder if GM and NASA will let us build the hot rod of Lunar Vehicles on Hot Rod Garage?
Although imagine a Hellcat HEMI powered Moon vehicle?
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