What happens to a car engine when giving max throttle while not in gear

Question

I was driving (well standing still actually) during rush hour, and I was suddenly wondering what would happen to a car engine when I would push the accelerator pedal all the way down while the engine is not in gear. Since most equipment these days is made pretty idiot proof I expect it wouldn't just blow up from the extreme RPM, but what would happen? Are there any detrimental effects? Can an engine survive these extreme RPMs, or is there a fail safe and would it just shut off?

(I understand this isn't really a car repair question, but from all the SO sites I think this one would be the correct one to get some insights in this question)

Answer 1

The RPMs would certainly increase but the mechanical components would be prevented from exceeding maximum RPMs by a rev-limiter.

Rev-limiters are built into the ECU (computer) of the car. When an engine is spinning at its maximum RPM set by the manufacturer, the ECU will not send a spark to particular cylinders to prevent the engine from spinning faster and potentially damaging itself.

If a rev-limiter were not in place the engine would have a high likely-hood of experiencing valve to piston contact. Valve to piston contact occurs when the valve cannot close quickly enough to get out of the way of the piston on an upstroke. The result is catastrophic with a hole being punched in the top of the piston by the lingering valve. Results of this type of failure will vary. Imagine loose bits of aluminum and steel flying around inside of an engine at or beyond its maximum RPMs and you get the idea.

This applies to modern cars with rev-limiters. Older vehicles will not have this self-protection device. I defer to an expert with more information regarding when rev-limiters began to be implemented by the manufacturers to edit this answer with some of that information and remove this paragraph. Thanks.

Answer 2

While most new electronically controlled vehicles probably do have a rev-limiter built in, this doesn't mean you cannot go past the red-line of the engine. The red-line is the theoretical maximum you ever want your engine to run at and is the limit at which your engine is designed to work.

Beyond this point one of the first bad things which occur is called valve float. This can happen because the valve spring does not provide enough control of the valve to close it quickly enough. This happens very quickly and very noisily. This will limit how fast the engine can go, ultimately, but this will happen past the red-line point. While valve float can occur at these extreme engine speeds, more than likely there will not be any valve to piston contact due to the fact the engine speed will decrease as soon as this point is reached. The main reason the engine will only go so fast as valve float is reached, is because compression within the engine is decreased, which means the engine will not function as efficiently, and therefor limits its speed. Other damaging things can occur as the harmonics within the engine are thrown all out of whack due to the valve float. As the valves do close, extra wear and tear can be expected on valves, valve seats, valve springs, and other valve train components. In the book Engineer to Win by author/racer/mechanic/genius Carroll Smith (p101), he states:

Simplistically put, under repeated (cyclic as opposed to continuous) stress the capacity of a metal to withstand stress gradually diminishes and, in most cases, cannot be restored. Metals which are subjected to fluctuating loads can and do break after a finite number of load cycles (of, more accurately, stress cycles) in which the loads applied and the resultant stresses imposed are always below the ultimate strength of the metal. This type of failure is termed "fatigue failure."

In other words, a piece of metal, lets say the valve, will live virtually forever in the confines of the parameters it was built to survive. Go beyond that point and it dies much quicker. This is seen at a logarithmic rate: the further you go past the designed point, the faster the failure occurs. This doesn't happen all at once, but it is accumulative and builds up over time. This not only applies to the valves, but to the entire engine as a whole. Any part which exceeds its specifications will ultimately die much sooner than it would have if it had never reached its maximum design. The engine can survive going past the red-line, but it's not something which should be imposed on an engine very often.

This leads us to your original question, the fact of whether stabbing the pedal to wide open throttle (WOT) with no load on the engine will cause problems for the engine. This depends on how long you hold it there. As already established, the longer you hold it the its running maximum (whether with or without a rev limiter), the sooner the engine will die ... and have no doubt, the longer you hold it there, the sooner it will die. If you stab the go pedal and quickly release it as it reaches its maximum, the engine will probably suffer no ill effects, but you've just added made it so the engine will live that much shorter of a life span. Remember, like any good chain, it's the weakest link which determines how well it will hold. Any single piece of an engine can fail, causing the entire thing to go up in smoke. That weakest link may be the valve, connecting rod, piston, or any of the other pieces in the engine. It just takes one to fail to make it all become a useless pile of metal.