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Low gear on descents & engine braking

July 10, 2017, 06:12:08 pm

1998 U-320 Cummins M11 Celect Plus 450 HP with joystick retarder.

Our unit is still new to us so I haven't done mountain descents yet.  I don't like reinventing the wheel with things like that.

With the gas engine trucks I've driven putting the transmission in a lower gear such as 2 or 3 would be enough to have the engine compression keep the speed you established before starting the descent.  So if you started the descent at 35 MPH, kept the transmission in 2nd gear, then the engine would maintain 35 MPH with either no, or very very little, braking required.

Will the Cummins M11 Celect Plus and Allison transmission work the same way, or does the diesel engine have less ability to hold the speed lower than a gas engine?

Thank you,
Chris

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Re: Low gear on descents & engine braking

Reply #1 – July 10, 2017, 06:28:56 pm

Gasoline engines
The term 'engine braking' refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released.

When the accelerator is released, the butterfly valve that controls intake airflow (the throttle valve) is closed and the air flow through the intake is greatly restricted (but not cut off completely). This causes a high manifold vacuum which the cylinders have to work against - this saps energy and is where the majority of the engine braking force comes from. The concept can be illustrated by the amount of effort required to blow/suck through a narrow tube vs. a wider one. It is the work the engine has to do against this restricted air flow that provides the braking effect.

While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction.

Diesel engines
Diesel engines do not have engine braking in the above sense. Unlike gas engines, diesel engines vary fuel flow to control power, rather than throttling air intake and maintaining a constant fuel ratio as gas engines do. Because they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects. This is partly why non-turbo diesel engined vehicles can coast in-gear for longer than an equivalent gas engine.

The higher compression ratio in diesels means they are harder to start, but once they are running the energy expended in compressing air is regained during the expansion stroke when the compressed air is allowed to 'spring' back, so the higher compression ratio doesn't cause any engine braking.

Above excerpt courtesy of Wikipedia:  Engine braking - Wikipedia

See also:  Retarder - Wikipedia

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Re: Low gear on descents & engine braking

Reply #2 – July 10, 2017, 07:01:53 pm

Ok.

So in the case of our engines, does that mean it is the gears/transmission that is maintaining the lower speed of the coach during a descent?

I know the general rule is "the gear used going up is the gear used going down".  But I like to understand a little more about what the transmission, gears, and engine are doing and why we are selecting a certain gear.

The M11 Select Plus is speed governed at 1800 RPM for automotive purposes, and 2,000 for vocational.  The peak torque occurs at 1,200 RPM.  So I see the working range as 1,200 - 1,800 RPM.

Would manually selecting a gear that keeps the transmission in the 1,500-1700 RPM range let the transmission/gearing do it's thing to keep the speed down, and then you apply retarder if the speed climbs above the target speed you are trying to keep?

Sorry if these seem like basic questions.  Our driving to this point has been flat driving where I can keep the transmission in "Drive" or "Mode" and apply the retarder to bring the coach to a stop, rather than applying the retarder to help control speed.

Thanks,
Chris

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Re: Low gear on descents & engine braking

Reply #3 – July 10, 2017, 07:14:51 pm

I drove my 4107 bus (Detroit 8V-71) for years with only the service brakes and engine braking. Paying close attention to speed, brakes and gears, I never had a problem even in the Rockies but the Jake makes even the steepest decent a piece of cake with our U300.

Chris,

No two grades are the same. Check air pressure at the top of the grade and gear down to a lower gear than you think you will need to be safe. Two lane highways are usually much steeper than interstates. Use pullouts to cool the service brakes. Don't let cars behind dictate your decent speed. I like to let the RPM build from about 1600 to 2000 RPM, use the service brakes to bring the RPM back down to 1600 and then repeat the procedure. Doing this too often means you need a lower gear. If you have a retarder, use it but remember it heats the transmission so watch the tranny temp. Be conservative the first few times as descending a grade is no place for mistakes. Once you get the feel of it, you won't have a problem and can enjoy the way down.

And no, I never ever use constant pressure on the brake pedal all the way down a grade. That's a good way of getting brake fade without knowing it.

Pierce

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Re: Low gear on descents & engine braking

Reply #4 – July 10, 2017, 07:18:43 pm

The celect plus m11 450 hp was/is a 2100 rpm under load rated motor as far as I knew.

I have run ours down  grades at 1800 to 2000 many times.

The internal engine frictions (?)seem to help the retardation effects and cool the trans more.

Noticeably less temp and less retarder needed so I changed from a higher gear and more retarder.

Plus had the brake pins checked, helper springs installed and the front rotors turned to true them up.

The service brakes on ours definitely cold blooded.  Not sure if the pads are oem Foretravel or not.

Versus the softer pads?  What has everyone else noticed or seen on changes?

The retarder seems made to compensate for the harder pads. 

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Re: Low gear on descents & engine braking

Reply #5 – July 10, 2017, 07:41:42 pm

I like to keep the rpms up as high as possible when using the retarder on a big decent to help keep the transmission temp down.

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