 | Home > Puma (Tdci) > Engine braking vs brakes |
 
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| Cheshire110 Member Since: 26 Jul 2013 Location: Cheshire/London Posts: 2754  
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You can certainly get he engine braking you need without being at 3500?! Surely 3rd at 2500 with a dab of brakes would be best?
I'd say a combination of both must be best, but don't really have any exact experience of icy driving. Does your vehicle have ABS/TC? Someone else may know better but 3500rpm seems excessive to me, and can't do fuel consumption any good either! Cheers, David Land Rovers of all shapes S3 onwards… Daily is a 110 V8. |
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1st Sep 2013 10:32am |
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| Sockpuppet Member Since: 17 Sep 2011 Location: Leicester Posts: 479
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Bear in mind that Diesels don't generate much in the way of engine braking compared to a petrol due to there being no throttle. 3500 rpm isn't excessive as the engine is being driven externally, it won't be using any diesel.
If you do this a lot you could think about adding an exhaust brake, I probably wouldn't add one behind the DPF if you have one though. |
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1st Sep 2013 10:34am |
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| limbrickh Member Since: 05 Oct 2012 Location: Canterbury Posts: 20  
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Thanks for responding so quickly David!
Fuel consumption on the way down is amazing! I'm pretty sure fuel usage is almost zero - which it defintiely is NOT on the way up 
I find that in 3rd it tends to run away a bit too easily. I have a Series IIa with a 6 cylinder Nissan LD28 and it has much better engine braking than the latest 2.2. I use 3rd when coming down in that but the 4 cylinder 2.2 doesn't seem to hold the vehicle back as well. Cheers Hamish |
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1st Sep 2013 10:38am |
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| limbrickh Member Since: 05 Oct 2012 Location: Canterbury Posts: 20
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Hi Sockpuppet
Yes, it seems to be very frugal on the way down despite the high revs. I always thought diesels had MORE engine braking than petrols due to their high compression - but then again, I don't really know that much about engines. I hadn't heard of exhaust brakes before but unfortunately I do have a DPF. Thanks Hamish |
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1st Sep 2013 10:44am |
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| Sockpuppet Member Since: 17 Sep 2011 Location: Leicester Posts: 479
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Exhaust brakes are fitted to most lorries in Europe for these reasons, they add extra engine braking. Bear in mind these engines are warrantied for a million km they don't seem to do any damage.
Diesels have less engine braking as regardless of how far you have your foot planted it will always take a full gulp of air. In a petrol you take your foot off the accelerator and the throttle closes and creates massive vacuum pressures which aid engine braking. You may well be able to fit one after the DPF, just I've never worked with them before so I'll err on the site of caution. |
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1st Sep 2013 10:54am |
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| limbrickh Member Since: 05 Oct 2012 Location: Canterbury Posts: 20
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David - sorry I didn't answer your question re TC/ABS.
Yes I have TC and ABS but the reason I'm avoiding the brakes is more the long term overuse problem rather than for avoiding a skid. I'm very conscious of not letting speed build up because, as much as ABS is amazing - if you meet sheet ice, speed is your enemy...especially when the drop either side of you is 500metres straight down. As an aside...about 5 years ago we were coming down the mountain and I watched a 10 metre bus do a 180 spin in front of us on a road that was 10 metres wide with cliffs on either side (the road is on a ridge). It ended up blocking the road so I had to apply the brakes - we ended up doing a 180 degree spin too with our back axle of the edge of the road and a very long drop below us. That was in the Series IIa with no ABS. Despite my back axle hanging off a cliff, I had a bunch of the passengers from the bus running over to me begging me to take them down the mountain - they weren't getting on that bus again! |
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1st Sep 2013 10:56am |
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| ericvv Member Since: 02 Jun 2011 Location: Near the Jet d'Eau Posts: 5816  
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That should be correct. A 2.4 Puma has compression 17.5:1 which is considerably higher than the 10:1 or less of a petrol engine, and engine braking is reflecting that. I believe the 2.2 Puma has a bit less compression, believe it is 15.5:1, and I have been driving one not so long ago and noticed the difference in engine braking. I think the right thing for you to do is to go down the mountain in a low gear which keeps the engine revving not higher than 3000 rpm, you may help with some pulse braking from time to time to keep it below that, but if you have to help with the brakes all the time, better to select one gear lower.  You never actually own a Defender. You merely look after it for the next generation.
http://youtu.be/yVRlSsJwD0o https://youtu.be/vmPr3oTHndg https://www.youtube.com/watch?v=_GtzTT9Pdl0 https://www.youtube.com/watch?v=ABqKPz28e6A https://www.youtube.com/watch?v=rLZ49Jce_n0 https://www.youtube.com/watch?v=XvAsz_ilQYU https://www.youtube.com/watch?v=K8tMHiX9lSw https://m.youtube.com/watch?v=dxwjPuHIV7I https://vimeo.com/201482507 https://www.youtube.com/watch?v=ZSixqL0iyHw |
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1st Sep 2013 11:20am |
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| Cheshire110 Member Since: 26 Jul 2013 Location: Cheshire/London Posts: 2754
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Interesting to hear people's advice / knowledge - learned a lot. Sorry if my advice was less useful than others'!
I guess slower = better and if it feels unsafe it probably is! I guess if it were me I'd use a combination of engine and foot braking. Out if interest, what is best to do when wanting to stop quickly on icy surface? (Other than pray!) Cheers, David Land Rovers of all shapes S3 onwards… Daily is a 110 V8. |
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1st Sep 2013 11:31am |
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| ARC99 Member Since: 19 Feb 2013 Location: North Yorkshire Posts: 1831 
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"I have noticed however, that the engine temperature is right at the bottom of the scale when I do this. Obviously the engine is not generating much heat because the fuel cut-off is starving the engine - as it should do. In addition, we usually have the heater on because the kids are cold after skiing."
You could always fit a radiator mufler to stop the cold air passing through the radiator. Don't make old people mad. We don't like being old in the first place, so it doesn't take much to  us off.
Richard |
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1st Sep 2013 12:01pm |
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| leeds Member Since: 28 Dec 2009 Location: West Yorkshire Posts: 8581
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Vehicle brakes slow down the revolutions of the wheels.
The brakes are only effective if the coefficient of friction between the tyres and the ground is good and high. The coefficient of friction of ice (also of wet grass) is to put it mildly and politely not a lot! IF you want to stop quickly on icy surfaces and brake hard, yes you will stop the wheels turning but will skid. Proper winter tyres will help, studded tyres also ( not sure of legality of them in UK), or chains. Best drive to conditions prevailing, I.e. suspect ice slower then normal, also do not drive where everybody else has. That broken ridged ice in gutter will give you more traction then the icy ruts where everybody else has been driving. Or taken a Scandinavian winter rallying course 
Brendan |
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1st Sep 2013 12:01pm |
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| landybehr Member Since: 17 Apr 2013 Location: -D- Posts: 173 |
If the petrol engine on it´s first stroke cannot breathe in because a throttle disc acts against that on run-over, the second stroke of the engine has nothing to compress. So one might have to compare the effect on enginge braking of a) creating a vacuum with a petrol engine and b) having to compress air. This is where I seem not to have listened to a teacher at some time. I´m inclined to think that compression is harder to do - definately when the compression ration of the diesel engine is about times 1.5 that of the petrol engine.
Now having driven a 2.2 as high as to 2600m I wasn´t very impressed by the engine braking too. The lower c/r of the engine can explain that. Depending on the roads I had to assist every few seconds with the brake, but as there was only very very little pressure to the pedal needed, I decided to get just live with it. |
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1st Sep 2013 12:02pm |
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| K9F Member Since: 12 Nov 2009 Location: Bournemouth Posts: 9610 
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The 2.2 does indeed have a marked difference in engine braking capability when compared to the 2.4. Realistically I think driving style has to be adjusted to suit perhaps?
From an older thread reference the 'rev hang' characteristic of the 2.2..... If you go through life with your head in the sand....all people will see is an ar5e!! Treat every day as if it is your last....one day you will be right!! |
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1st Sep 2013 12:03pm |
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| landybehr Member Since: 17 Apr 2013 Location: -D- Posts: 173 |
If driving conditions change a lot, resp. driving up and down all the time, I would not consider the radiator muff. In this case there seems to be a continuous and long descent; and while the engine is on overrun and shouldn´t inject significant amount of fuel - I fear the engine will still not produce heat that could be "saved" by the muff.
? Fuel-to-air heater (Websto etc.) ?? |
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1st Sep 2013 12:06pm |
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| Sockpuppet Member Since: 17 Sep 2011 Location: Leicester Posts: 479
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True but when you compress a gas it gets hot, very hot. Hot things especially gases want to expand so on the down stroke actually puts pressure on the piston. On a petrol the 4 stoke cycle under engine braking is. - Suck Nothing In (Vacuum Formed) - Compress Nothing (N/A on braking effects) - "Bang Nothing" another larger vacuum formed. - Blow nothing - (N/A on braking effects) On a diesel 4 stroke the cycle would be - Suck Full Air (N/A On Braking) - Compress Full Air (Engine Braking) - "Bang" Full Air (Air will expand and return most of the energy of above to engine) - Expel Full Air (N/A On Braking) THis is why cummins used to fit (may still do) jake brakes to engines so that at the top of the compression cycle the pressure was dumped and a vacuum created helping engine braking. |
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1st Sep 2013 12:30pm |
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