What’s it’s role in driving a truck? When I drive a car I use engine braking quite a bit when approaching junctions and obviously going downhill. Do you need to adjust your use of engine braking when driving trucks?
This from a newbie
I use engine braking the same way as in a car, but some of the newer Dafs seem to have an exhaust brake that comes in as soon as you take your foot off the loud pedal, so you get quite a marked slowing down effect until you get the speed down. Truck braking seems to be quite an involved subject as you can see from other threads.
Gordy
The Idea is to allow you to slow down or maintain a speed down hill with out resorting to the brakes.
I know nothing about anything mechanical, but as I understand it…
There are two main kinds. Retarders which work on the drive shaft,( I think) & exhaust brakes. Neither should be used in icy conditions as they can cause the vehicles wheels to loose traction & thus you loose control.
Many modern trucks have a three position switch which allows you to either apply retardation along side the use of the brake, when you lift your foot off the accelerator or not at all.
The difference between a Truck & a Car is simply weight & the fact that it pushes you. So on long down hill runs it is much safer & sympathetic to your vehicle, to keep your revs up & use the retarder or exhaust brake than constantly applying the brakes, which can cause overheating & thus brake fade.
I’m sure someone will coreect me & tell you the correct infomation, but as I said I’m no wanna be mechanic.
taxman:
What’s it’s role in driving a truck? When I drive a car I use engine braking quite a bit when approaching junctions and obviously going downhill. Do you need to adjust your use of engine braking when driving trucks?
I would guess here Taxman, that you did your car test ‘some time ago’?!! Nowadays students are taught to ‘slow the vehicle down using the brakes and then select the correct gear to match the road speed’. This is the DS answer whilst a learner driver in any Cat.
Things change a bit in the ‘real world’ as we then get into the realms of retarders, engine brakes and the reality that re-lining your brakes etc is expensive!!
One of the main problems is that the UK standards are too outdated.
Examples:
Drivers are not allowed to cross their hands when steering - this is from when power steering was not found on cars and it was safer to ‘push-pull’ the steering.
Change down through the gears as you approach (i.e.) a stop sign - this is to keep momentum in the wheels (prevents wheellock) and to keep the engine and services that run from the engine (brake servo) working correctly.
Reality:
Here in Germany drivers are allowed to cross their hands as modern cars and power steering has made this a safe option. If anyone has done any Advanced Police Driver Training then you will have been taught to cross your hands at times!
There maybe some points in here that not everyone agree’s with but it is the ‘modern’ way of thinking!
Again, in Germany (they review the standards often enough to be up-to-date!!), as you approach the stop sign etc you put the car into neutral and coast to a stop. This, supposidly, saves no end of fuel and is therefore ‘an economical way of driving’. The issue of brakes and momentum doesn’t apply on modern cars due to ABS etc etc.
the down-thru-gears to a stop thing…
now they teach block changing on the downshift, so you’re expected to come up to junction in high gear, select driving gear as if to go when around 1 to 3 truck lengths from junction (depending on how open it is) and slowing on brakes rest of way using engine as a safety device to control speed on downward curve on approach should something fail, and also for instant drive when you can see its clear to go. Its also more economical than using every gear, as the revs are lower when in the higher gear on the slow-down. Many trucks also have an automatic engine-brake system that comes in on braking if above a certain rpm and speed (see scanias) and so to make the most of braking available, a gear should be selected. saves pads/drums/disks.
IMO the only time its safe to coast in neutral is when you’re doing walking pace, holding on the brake and its a case of visibility or depth of traffic. the engine is a back-up or safety secondary braking system.
dipping the clutch in and coasting aint so bad in the situation you mentnion but nudging neutral loses too much time if its clear to go, when you’ve got a 45 foot trailer to move round, and something with a slow gearbox like a scania.
but then, I’m a novice so probably not right in some regards 
el gordo,
By way of clarification, although I am a PSV, HGV and car driving instructor, I was actually describing Cat B students i.e. car.
Obviously, the comment about selecting neutral doesn’t apply to either PSV or HGV.
I’d personally alter the speed on the brakes, unless your going down a gradient, when I’d use engine breaking. The problem with not using it on a long downhill gradient is the term break “fade” this in fact means no brakes 
.An ex gaffer once said it’s cheaper for him to re-line the breaks (drums) or put a new set of discs in than it is to put a new gearbox on every five munites because it’s shafted through unsympathetic driving (those wern’t his exact words 
). I would guess that using the breaks and block changing the gears would also be better for fuel consumption. Personally I wouldn’t fancy changin down through however many gears (worst case secnario) every time I stopped either . Also with some of the coputer controlled gearboxes as for the Merc Actros’s, they select the gear they think you’ll need for the speed your doing, so using engine braking would be difficult to say the least .
Cheers
Ian.
dunno much about it but Scania’s retarder is excellent
It’s worth mentioning that a naked Diesel engine has no natural engine braking effect - it doesn’t continue to compress when you’re off the accelerator (actually, more of a vacuum really - for the pedants among you! ) like a petrol engine does.
Therefore, to get any efficient engine braking you have to add a retarder device - either electromechanical, like a Telmar, or something compressor-based, like the well-known Jacobs Engine Brake or Jake Brake as it’s commonly known. The Jake Brake literally turns the engine from a power generating source, into a inertia absorbing source - basically a compressor. It’s this compression which gives you that strong engine braking you get with a Jake. Plus, they make a great noise! 
So, if your truck isn’t fitted with a retarding device you may as well use the service brakes. They are going to be much more efficient than the minuscule amount of retardation you’d get from a naked Diesel engine - and without the risk of over-revving the thing while you try and get some ■■■■■■ effect!
Actually, diesels certainly do continue to provide compression on the over-run (more so than a petrol engine). The problem is that this is countered almost 100% by the “rebound” on (what would be) the power stroke.
{a pedant writes…}
Roymondo:
Actually, diesels certainly do continue to provide compression on the over-run (more so than a petrol engine). The problem is that this is countered almost 100% by the “rebound” on (what would be) the power stroke.{a pedant writes…}
lol - yes - I know, it’s the lack of vacuum in the induction which basically nullifies any braking effect… Which is why I said:
Sir LANs-a-lot:
… (actually, more of a vacuum really - for the pedants among you!) …
Anyway - pedantry aside - Diesel’s certainly do need assistance to provide decent engine braking… And I still think Jake’s make a great sound! 
Sir LANs-a-lot:
lol - yes - I know, it’s the lack of vacuum in what would be the power stroke which basically nullifies any braking effect…
Why is this different to a petrol engine?
MrFlibble:
Sir LANs-a-lot:
lol - yes - I know, it’s the lack of vacuum in what would be the power stroke which basically nullifies any braking effect…Why is this different to a petrol engine?
That should read induction - not power stroke.
The difference being, in a petrol engine - when you have no throttle - the air intake is effectively blocked. Not completely blocked, obviously, but enough to create a vacuum in the engines induction stroke. This vacuum creates engine braking.
By comparison, when a Diesel has no throttle it still ■■■■■ air on induction - therefore any braking effect created by induction vacuum is lost.
Although that would be partially mitigated by the fact that a petrol engine then wouldn’t have to pump air out on the exhaust stroke, whereas a diesel engine would have to pump against the exhaust system.
You certainly do get a reasonable amount of engine braking (esp. at higher revs), just from friction in the engine - there’s an appreciable difference between running with the clutch down and with the clutch up.
It’s getting toooooo technical, my head hurts 
.
MrFlibble:
Sir LANs-a-lot:
lol - yes - I know, it’s the lack of vacuum in what would be the power stroke which basically nullifies any braking effect…Why is this different to a petrol engine?
In a petrol engine, the throttle is closed thus preventing (much) air from being drawn in. In order to spin the engine over, on the “induction stroke”, the turning force applied through the crankshaft has to pull the piston against this vacuum (or to more accurate, against atmospheric pressure at 14psi acting on the underside of the piston).
The piston is then pulled back by the vacuum (or again, more accurately, it is pushed up again by atmospheric pressure) on the “compression stroke”, cancelling out the effect described above. But then on what would be the power stroke, the sucking against a vacuum scenario is repeated. As a result, spinning the engine over takes a fair amount of effort.
In a diesel, the “throttle” remains open (diesels are controlled by adjusting the amount of fuel injected, not the air), so the initial induction stroke is relatively easy.
The full (of air) cylinder is compressed on the compression stroke (which provides engine braking) but then “bounces back” on the “power” stroke, negating the braking effect. on the "exhaust " stroke the air is simply blown out through the exhaust., which again doesn’t take a great deal of effort.
The “exhaust brake” on a truck works by restricting the flow in the exhaust system,.
Yes, I understand the role of the exhaust brake; it was the “power stroke” typo which confused me, as any gain/loss in the power stroke is compensated for in the compression stroke.
I didn’t realise that diesels didn’t restrict the air flow on the induction - does the fuel/air ratio not matter on a diesel then?
Mr Fibble:
I didn’t realise that diesels didn’t restrict the air flow on the induction - does the fuel/air ratio not matter on a diesel then?
Roymondo:
In a diesel, the “throttle” remains open (diesels are controlled by adjusting the amount of fuel injected, not the air), so the initial induction stroke is relatively easy.
Pay attention the back 
With a diesel engine they have a much higher compression ratio than a petrol which gives you natural engine braking provided you are in the correct gear
A jacobs brake on an engine closes half of the valves which increases the effect.
A retarder works on the propshaft or gearbox and increases the power needed to turn the shafts thus slowing you down
An exhaust brake is a simple butterfly in the exhaust manifold restricting gases that also slows the engine slightly.
In the olden days You were taught to drive down a hill in the same gear as you would use to go up it.
My motorcycle is a V Twin and the advantage of these, is that as you roll off the throttle, it slows down. this doesnt happen with an inline four as much.
Thats a very simple explanation
This brings back memories of being told as a novice car driver not to switch off the engine while coasting down a long hill. Those that tried it were most surprised when a massive backfire blew their exhaust to bits when they re-started their engines due to petrol being sucked into the engine and gathering un- burned in the exhaust.
Gordy