Posting this from my blackberry. Okay not really.
If your car is accelerating out of control, and you shift into neutral, will there be any damage as the engine struggles against no load, or will the rev limiter prevent all damage?
Suppose your car is functioning normally, and you are driving on flat ground, meaning that all acceleration is due to your throttle position. In this case, if you accelerate to the rev limiter, will there be any damage?
I do that all the time with all my cars and no damage so far. Of course it is not a good idea to let the engine at redline for prolonged time but a few seconds every now and then isn’t going to do any harm.
Under those conditions - presumably wide open throttle and sudden loss of load - I wouldn’t count on a rev limiter to prevent any damage. It might work to prevent it, but that’s not really the situation it’s designed for. I also wouldn’t count on a given vehicle even having a rev limiter unless I knew with certainty that it did.
Why not just turn off the ignition switch?
Get that wrong, and you’ll engage the steering wheel lock. Some cars are overly tricky about that.
And keep in mind that throttle stuck open = no manifold vacuum = no brakes.
When I get my F350 PowerStroke Diesel emissions tested, they have me put it into neutral, and then depress the accelerator pedal to the floor, and hold it for a few seconds, repeated 4 times. I hate doing this, but it hasn’t destroyed the engine so far.
My old car had a rev limiter at 8000 that I hit a few times (it was a 5 speed) but what it did was cut off the fuel flow and then turn it back on , it was more to warn me than to do anything else. I never kept the car above 8000 to see what would happen, I did not want to see the results.
Do cars now still lock the wheel when you turn off the power? I think that is mainly if you take the key out on some cars.
Really? I would have thought that a rev limiter would be common on all cars with a computer. If revs == really high then don’t inject fuel.
My question was inspired by the news of all the Priuses run amok: http://www.seattleweekly.com/2009-04-22/news/the-flip-side-of-the-perfect-prius/
either blasting forward uncontrollably, or the brakes failing.
Loss of manifold vacuum implies loss of the vacuum boost (aka power-assist) - but the brakes still function.
There is no way they are going to sell cars that have no brakes if the engine dies. I have push started cars and the brakes work fine before the engine starts. You just need to press down harder on the pedal.
Not exactly answering the OP, but what is going to do less damage? Careening down the road under full throttle and smacking into a brick wall / road divider / semi truck at 80 miles an hour, or shifting into neutral and having your engine blow?
I know which I would rather choose…
What actually breaks when an engine overrevs anyway? Valve float causing them to hit the pistons? If you’re lucky that might only bend the valves.
This once happened to me – while driving an old truck (1973 Sierra in 2003), the [throttle cable?] in the carburetor got stuck wide open after I floored the gas pedal to get away from a developing situation. The engine was rapidly exceeding the ability of the brakes to keep the truck down to rush hour speed, and I shifted into neutral while coasting to the side of the freeway. By the time I stopped on the shoulder of the road, the engine was on fire.
Happened to me while driving a race car. I braked to slow the car, killed the ignition, and coasted to a stop.
Depends on the engine. Most modern engines have non-interference valves - the piston wouldn’t hit them even if then stayed stuck open.
I’m guessing that the connecting rod would fail and subsequently destroy the crankcase.
If you have hydraulic lifters…otherwise you’ll break a spring or retaining clip or the valve itself. Then bad things happen. If the valve train holds up you might spin a bearing and cash-in the crank.
Many new cars have interference engines such as all Hondas and Nissans. Pretty much all diesels are interference too.
I know someone who had this happen and it completely wrecked the engine.
His car is a diesel with a turbo, n oil seal blew so the engine was sucking in lub oil into the turbo, this went straight into the cylinders, and being a diesel it used the mixture as fuel. Problem is that there was unrestrained amounts of fuel/air mixture going in and no way of controlling i. Turning off the ignition is no help on a runaway diesel engine since it works on compression, and the fuel was not going in through the injection.
He put it into neutral and the engine simply over revved and destroyed itself. In retprospect it might have been better to jam the brakes on change to top gear and try to stall the engine, however when you have only a few seconds and you have no idea what is happening, hindsight is a wondeful thing.
While I agree that I would not count on a particular engine on having a rev limiter unless I knew it was there for a fact, I would disagree about how well they function. I used to regularly demo the rev limiter function for my students. The engine isn’t very happy about it, but I never blew one up.
This is incorrect. The vacuum system has check valves to prevent loss of vacuum during full throttle and engine off conditions. Even if those fail, you mechanical brakes still work, but will require more effort.
The point of failure will be different for different engines. On one engine it could be a valve failure, on another a connecting rod failure, on a third a timing belt failure.
Unless you have a cite to back that up, I would disagree with this statement.