[SystemSafety] Yet another supposedly-runaway car

michael.ellims at tesco.net michael.ellims at tesco.net
Fri Mar 8 11:51:33 CET 2013


> How about Neutral?

Neutral would do it, however I strongly suspect the significant point is that in an emergency in a vehicle with an automatic transmission it might be difficult e.g. nasty grinding noise (or similar, I’m not sure) which would suggest to the vast majority of people that something was amiss and that it wouldn’t or didn’t work.

> Would that stop the engine fighting the brakes and make it much easier to stop?

Yes, putting the engine into neutral would of course make it easier to stop. However the brakes should easily overpower a normal engine, on the vehicle in question develops 131 ft/lb of torque the front wheels all up should be capable of somewhere in the region of 6-7000Nm (i.e. more than 4400 ft/lbs). The gear box will ramp that up a lot in first gear, e.g. to at least 1000Nm (necessary for curb climbing at 0mph) but we’re talking high speeds which implies high gear ratios.

There is of course the issue here that again the vast majority of drivers don’t use anything like the braking capacity of the vehicle. Most (about 98%) braking is below 0.3g in normal driving, the legal minimum the vehicle has to be capable of is around 0.6g and most vehicles can manage around 0.8g. At 0.6g your literally hanging in the seat belt. Again that is scary and by itself might frighten most people.

> Would some limiter stop the engine over-revving and doing damage?

If one was fitted, it’s normal for large diesel engines and I know at least one road vehicle has it fitted but in a petrol engine is dependent on being able to control the throttle. Cutting the sparks would get to the same point, not sure about the this though  as pumping a raw fuel/air mixture into a turbo or catalytic converter is probably not a very good idea.


----- Original Message -----
From: Martyn Thomas <martyn at thomas-associates.co.uk>
To: systemsafety at techfak.uni-bielefeld.de
Sent: Fri, 08 Mar 2013 09:40:57 -0000 (UTC)
Subject: Re: [SystemSafety] Yet another supposedly-runaway car


> On 3/7/13 6:44 PM, michael.ellims at tesco.net wrote:
>>
>> Peter wrote: “turn off the ignition" or "force it out of gear / force 
>> the automatic-shift lever into "Park".
>>
>> NEVER put a moving vehicle with an automatic transmission into 
>> Park!!!! This will lock the rear wheels as P engages the transmission 
>> lock. At speed this will result in the vehicle spinning. This is also 
>> probably impossible as you would have to go though Reverse to get 
>> there which would do the same thing – the gearshift is called a PRNDL 
>> for a reason ;-) 

How about Neutral? Would that stop the engine fighting the brakes and 
make it much easier to stop? Would some limiter stop the engine 
over-revving and doing damage?

Martyn
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