Over-run/engine braking

Post by **MacAoidh** » 23 Oct 2006, 10:22

Just read elsewhere that over-run is when I take my foot off the throttle and the momentum of the vehicle turns the engine.

I've been wondering for a while whether fuel is used in this situation? When particularly skint, on the big downhills around here I've often tended to knock it into neutral. Wind resistance then provides adequate speed control.

Secondly, I was being towed a while back, very short rope, rather fast tower (who I'd only met a couple of minutes earlier...). I was making a lot of use of the brakes, to keep the rope short, was worried about over-heating them, so I engaged the engine in 4th gear while not switched on. Just a bit, at the end. Was that smart, or unsmart? (When we arrived, there was a very bad brake smell!)

Anyone know?

Curious, of Hay.

Post by **stuckin88** » 23 Oct 2006, 19:26

if your foot is off the throttle theres no fuel bein used--its not safe practise to go down hill in Irish overdrive--yoove got no engine brakin so yer basically out of control---as fer yer towin-its not summat yer do every day-so just arrivin safely at yer destination woz smart enough---

Post by **kevtherev** » 23 Oct 2006, 21:18

I like to coast too...safeley..whenever possible...it's quieter

I'll sometimes put the mechanical brake on a few notches too,

for short very steep hills..limits fade

Post by **ghost123uk** » 24 Oct 2006, 08:44

Downhill, in gear, foot off, will use the same amount of fuel as at tickover, maybe a touch more as the revs will be higher.

Coasting downhill out of gear - well whilst I have done it, it cannot be recomended as you have no engine braking, so your brakes are going to get overworked, and on top of that, if the engine is not running, your brake servo will fade away and you will have to stand on the brake peddal to get any effect !!

Re towing - always a risky business - especially if one or both of you are not used to it. Communicating you situation via the mobile phone is a very good idea here, such as "SLOW DOWN you daft b*stard" or "the junction ahead is clear" etc, tho I suppose one should be using a "hands free" kit these days !! ( We used to use our CB Radios for this )

Re using the engine as a continous "brake" but not actually running - Ummmm - can't see any real problem there, in fact on the face of it, it seems a good idea. Though other may have some input here...

E D I T = hang on - using an non running engine as a brake will put unburnt fuel into the exhaust system !! - I retract my statement that it might be a good idea !!

Post by **MacAoidh** » 24 Oct 2006, 17:01

Cheers all,

Always worth reiterating the dangers of doing without engine braking, but yes, this is just referring to situations where the air brakes are more or less enough. If I suspect brake fade could be an issue I'm always ready to knock it into 3rd, pull over and cool down etc.

And interesting to hear about engine braking without combustion. Does that xplain why the thing wouldn't start afterwards, until it was pushed off a big hill?

Post by **ghost123uk** » 25 Oct 2006, 10:33

MacAoidh wrote:Cheers all,
And interesting to hear about engine braking without combustion. Does that xplain why the thing wouldn't start afterwards, until it was pushed off a big hill?

I reckon so.
Probably flooded.
If one was to do this with a petrol engine, or even worse an LPG powed one, the exhaust might fill up with explosive gasses and god knows what might happen when it fired up !!!

:run

Post by **toomanytoys** » 25 Oct 2006, 11:07

If everthing is working correctly then with the engine off (and ignition off.. but very unsafe to do this due to steering lock) a very minute amount of petrol will be drawn in as the "stop" valve on the carb would not be energized, similarly a Fi engine would not have fuel pumped in..
LPG isnt a proble either, no gas is dispensed if the ign os off, or the engine is not running for more than 3 seconds.... if the ignition is on, then "idle" combustion will take place in any case..

Post by **ghost123uk** » 25 Oct 2006, 11:25

Ah Mr toomanytoys, you are of course correct.

I was forgetting about the stop valve thingy.

Still, good point you made re the dangers of towing with the ing OFF and steering locks !! - I have seen that mistake done !! ( not by me I hasten to add )