Updated: Oct 16, 2021
There will be a slight thrust effect from a regular vehicles exhaust pipe, but it will be little and negligible. Almost all road car exhaust characteristics are not suitable for generating thrust and are only intended to remove the exhaust gases from the engine.
However, some vehicles, such as top fuel dragsters, actually use the exhaust to provide downforce. A top fuel dragster generates about 12,000 lbs of downforce at 325mph, 800-1,100 lbs of that downforce comes from the exhaust itself.
In this article, I will tell you how much thrust an exhaust generates, and how this is used.
Table of Contents:
Does a Cars Exhaust Produce Thrust?
Exhaust gases from an engine contain a considerable quantity of heat energy. The exhaust also comes out of the exhaust pipe at a high rate of speed, similarly to a jet engine, right?
In a way, yes. Due to the fact that gas is being expelled from the exhaust system, and in accordance with Newton's third law of motion, every action has an equal and opposite reaction, therefore a car's exhaust system does produce some thrust. (1)
When you push a cart, for example, the cart pushes back against you. The same is true of an exhaust. When the exhaust gases come out, it pushes against the car, providing thrust.
How Much Thrust Does An Exhaust Generate?
The amount of thrust generated from a normal passenger vehicle will be small and not noticeable, this is due to how an exhaust system works and its primary role to remove exhaust gases from the engine.
It is difficult to find out the exact amount of thrust generated from an exhaust simply because of how much variation there is between exhaust systems, engine rpms, power figures, etc.
Force = mass x acceleration is the calculation use to calculate thrust. However, we cannot work out the thrust generated due to the wide variety of vehicles.
Because a regular passenger car is highly fuel efficient, it will burn through much less fuel than a top fuel dragster would, for example. The engines in road cars are also much smaller and have a lower power output.
In addition, a cars exhaust system is not designed to generate thrust and has many restrictions, reducing the velocity of the exhaust gases. As shown in the image below.
Sometimes, the exhaust is even facing the ground, meaning there be almost no forward thrust produced by the exhaust.
One way to visualise how little thrust a regular exhaust provides is to start the car in an open and flat area, away from others, lower the clutch and rev the engine up a little bit (ensure the engine is up to operating temperature if you do this). The car will likely not move.
This means that the exhaust is producing too little thrust to move the car.
You may assume that adding a propelling nozzle to create thrust would be advantageous.
However, the best jet or rocket nozzles, on the other hand, are built with a considerable back pressure behind the nozzle, accelerating the exhaust gas as it passes through.
In a road vehicle, this is counterproductive since the engine must work harder to expel the gas, lowering efficiency and power output.
How Is This Used in Vehicles?
There is possibility of using the exhaust gas for useful purposes, such as the following.
The aerodynamics of the vehicle body may be enhanced somewhat by carefully putting it into the low-pressure region behind the car.
On the Bugatti Chiron Super Sport 300+, the exhaust pipes are placed differently than the standard models, decreasing drag and increasing top speed.
The exhaust placement does have an effect on aerodynamics.
Placing the exhaust in an upward direction, like that seen on top fuel dragsters, actually increases downforce without the need of other aerodynamic bodywork.
In theory, placing the exhaust system at the front of the car facing upwards would increase downforce on the front wheels slightly at any speed.
This effect can be used anywhere on the vehicle, although most engines and exhaust systems are not designed to do this and would have a minimal effect.
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1. Wikipedia. Newtons third law of motion. (Last updated in 2021, Aug*)
* Last updated at the time of writing this article.