Hi all, new guy here from Melbourne Australia. Been lurking on this site for awhile now and this thread has finally prompted me to join the forum. Thought I'd make my first post both an introduction and a contribution to the thread. Can trace my interest in LSR back to the age of five and have been studying and researching the technical aspects and the history of the sport since then. That said, I make no claim about being an expert on the subject, or any other for that matter, I have no formal training or qualifications in anything ( I left school young ). Also, I have no previous vehicles that I can point to as proof of any ability. As such you can make your own judgement on the technical merit of what I write. My opinions expressed are just that, they may or may not be fact. Shoot me down in flames if you can. Sorry to make this post rather long, I'll generalise to simplify it as much as I can. I hope it will be worth your while.
My aim with this post is to attempt to clear up some grey areas in the understanding of exhaust flow and how it relates to vehicle dynamics. It is true the exhaust contains considerable energy that is wasted and I haven't yet seen a vehicle take full advantage of this energy. As most everyone would know the exhaust flow can create thrust that contributes to pushing the car forward, this is true however not completely understood.
First, a brief refresher course. Rocket propulsion and Jet propulsion both come under what I term as a ' reaction engine'. That is, they propel themselves forward by the expulsion of matter (called the propellant) in the opposite direction. Newton's third law of motion states that ' for every action there is an equal and opposite reaction'. This matter could be in the form of a solid or liquid but is most commonly a gas produced by a chemical reaction in the combustion chamber. Rocket propulsion differs from Jet propulsion in so far as with a rocket, all of their propellants are carried on board the vehicle and the expulsion rate and force generated is largely independent of outside conditions. With a rocket propelled vehicle, the vehicles velocity has no relationship to the engines exhaust velocity. Rockets can, and do travel faster than their own exhaust velocity. With Jet propulsion it's slightly different. Only one of the propellants used in the combustion process is carried on board the vehicle, the other, Oxygen, is provided from the surrounding air. Jet propulsion works by adding energy to the gas stream flowing through the engine. As such a jet propelled vehicle cannot travel faster than it's own exhaust velocity.
At this point it's important I try and explain the difference between gross thrust and net thrust. Imagine a jet propelled vehicle held stationary with the engine at full throttle. In this condition gross and net thrust are one and the same. Now if the vehicle is let go and begins to accelerate two things happen. The gross thrust or 'overall' thrust will begin to rise (usually through a ram effect making the compressor more efficient and moving up through a pressure regime) however the net thrust, that which is actually propelling the vehicle will begin to fall. That's because the momentum added to the gas stream, the ratio of inlet velocity to exhaust velocity is reducing. With a Rocket, gross and net thrust remain the same irrespective of the vehicle's velocity.
Now lets apply this to a car or bike at Bonneville. The primary source of propulsion is with a piston or even turbine engine providing torque to rotate drive wheels. Sufficient traction provides the thrust to propel the vehicle forward. Secondary would be thrust generated from any rear pointing exhaust outlet. Being air breathing, the exhaust momentum would be classed as Jet propulsion and would conform to the above statement. I believe most vehicles at Bonneville would outrun their exhaust velocity and the net thrust at top speed would be a negative value. That's not to say it doesn't exist, your way of visualizing it just has to change. Up to the point of going negative it was providing more than what it was costing, now, when negative it should be seen more as drag minimization rather than ' free thrust'.
I'll touch briefly on the Meredith effect for radiators. The theory is that the air flowing through gets heated and leaves the duct at a higher velocity. In practice, through losses, there is no momentum increase and the net thrust is negative. Once again, the design is really based on drag minimization. However, if you were to merely quote the gross thrust output it would be an impressive figure.
So what can you do about this? First up, the exhaust has to point rearward and the velocity as high as possible to delay the point of going negative. Most exhaust outlets are way too big and velocity suffers as a result. The major pressure drop is through the exhaust port and the rest of the system is essentially ducting. Ideally you want a short pipe to keep losses to a minimum, direct it rearwards and then (now you can take this to the bank) put a nozzle on it. I'll avoid talking about the interaction between exhaust and vehicle aerodynamics in this post as it's growing way too long. My only objective here was to be thought provoking.
Cheers,
Dave.
