Eli5:how does petrol engines differ from diesel engine?if so then what type of engine is used in rockets?
In: Technology
In a gasoline engine, fuel and air are mixed and injected into the cylinder. The piston compresses them. Then a spark plug creates a spark which ignites the mixture.
In a Diesel engine, air is injected without fuel. The air is compressed to a point where it is so hot that fuel would ignite on its own if it were present. This would mess with timing and such, so the fuel is injected at the end of this compression stage. Because the air inside is so hot, the fuel ignites.
Diesel engines also have something called glowplugs. Because a Diesel engine requires the heat of compression to ignite the fuel, it can be hard to start an engine in the cold. Glowplugs are basically just heating devices to help get over the initial cold when starting.
Rocket motors are reaction engines. They produce thrust essentially by chucking all the burnt stuff out the back, *really* fast. The underlying principle is basically the same though — a fuel and an oxidizer are mixed, ignited (once, unlike intermittent automobile engines, rocket motors are typically continuous), and the reaction products are expanded inside a nozzle, which produces thrust.
Diesel engines use a ton of compression and then the “glow” plugs cause ignition. Petrol uses less compression from the pistons and doesn’t require much for ignition as the flammability rate for petrol is much higher than diesel.
Rockets these days use a lot of different fuel sources. Cryogenic helium is loaded just before takeoff. I think the rapid phase change for it from liquid to gas is what causes the propulsion.
Gas and diesel engines are use individual explosions to drive their cylinders. The main difference is that the diesel squeezes the fuel so hard that it explodes while a gas engine doesn’t squeeze quite as hard and (unless mistuned) needs a spark to set it off.
The combustion in a rocket, jet or turbine engine works more like a blowtorch or BBQ. Instead of individual explosions, fuel is constantly added to a continuous burn.
In a gas or diesel engine, the force of the combustion pushes the piston which turns the crank. In a rocket engine, the force of combustion pushes the hot gas through a nozzle which pushes the rocket in the opposite direction.
Both use a piston (I’ll leave the Wankel Engine out of this)
Petrol or gasoline engines work like this. The piston is pulled down to the bottom of the cylinder. During this process the cylinder is filled with a combination of air and fuel. The piston is brought back up to compress the mixture and ignite it with a spark from the spark plug. The Combustion forces the piston down and starts the process over.
Diesel works similar. However the combustion is caused by the air temperature after compression. As the air fuel mixture is compressed it heats up and then ignites.
Rockets use something completely different. They use Rocket Engines. Fuel and oxygen are combined and sent to a compression chamber and ignited. The exhaust is then sent through nozzle. That nozzle will help accelerate the flow. This produces the thrust needed to move the rocket.
Gasoline/petrol is ignited using a spark plug. A small electrical spark is generated in each cylinder during every cycle. Fuel goes boom, engine turns.
Diesel fuel is ignited by pressure. The cylinder compresses it hard enough that the temperature increases very quickly, fuel goes boom, engine turns. Some diesels use a glow plug, which helps the process happen a little more quickly and efficiently. Diesel is more efficient, as it contains more potential energy by volume than gasoline. This is why it is normally applied in huge engines where power to weight ratios are most important, like locomotives and semi trucks.
Rocket engines use different fuels. Liquid oxygen is one. The mechanism is very different though. In a rocket, the fuel is ignited, which makes it expand violently. The expanding gas goes out the bottom of the rocket, which pushes the rocket up.
Gasoline loves to get lit, the other needs to get forced.
In a gas engine the combustion area basically needs a spark on the fuel and air for it to burn. The negative of it being so easy to combust is all that energy goes away too fast and the engine is unable to fully use it. Hence why gas cars don’t feel as powerful and spend more fuel but feel smoother.
The Diesel engine takes fuel that has a lot of energy but doesn’t like letting it out so in the reaction area air gets squeezed so much it gets hot enough to make the fuel explode when introduced and because there’s so much force involved in the entire cycle that’s why Diesel cars feel very powerful from the start but also why they shake a lot, especially true with old or aging cars.
As for their sound, what we hear is mostly the gasses from the combustion coming out of the tail pipe through connected tubes to the end of the combustion area, the specific tones are due to the tube organization and the sequence of each reaction and how many combustion areas there are.
Rocket engines are much different but the concept is closer to a gas engine using a Diesel like fuel that doesn’t blow up easily (for safety) but needs to do it fast releasing as much energy all at once. Some of that energy is then directed through nozzles, what we see from the outside, the bell shaped thing. They are actually quite inefficient, a lot of that energy is turned into noise and other vibrations. It’s so much waisted energy that at launch water is pumped under the engines to muffle the vibrations otherwise it would rip apart the launchpad, the ship, and everyone within a certain distance and make many more deaf.
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