Tuesday, August 6, 2019

Geeky Driveline Info

I've downloaded some information on the Caterpillar C7 engine and the Freightliner XC chassis just so I can learn a bit of maintenance. The 7.2 ℓ turbocharged, direct-injection diesel engine is pretty similar to the 5.9 ℓ engine in our 2005 Dodge 3500 and is the smallest engine that Caterpillar actually manufactures. Similar in weight at 1300#, a bit louder, and a similarly shaped torque curve to the 5.9 Cummins. The operating RPM range is narrower (1440 - 2400 RPM) with the torque peak of 860 ft-lbs at 1440 RPM. Cold starting is the same with a grid heater in the intake manifold. I will probably look for a large vehicle oil change place once I get on the road. Oil capacity is either 21 ℓ or 30 ℓ. The C7 uses the HEUI (Hydraulically-actuated Electronically-controlled Unit Injector) system so clean oil is essential to trouble-free operation as very high-pressure engine oil is used to inject the fuel. The HEUI system provides only a single injection event unlike the multiple events with the common rail systems so the engine is noisier. On the other hand, extra fuel isn't injected just to improve emissions or quiet the engine during operation. My first observation when I heard the engine running was that it sounds like a diesel should.

The Allison automatic is a bit different from one the GM uses in their pickup with the Duramax. Maybe it's just a different model. It has 6 speeds forward, neutral and reverse. No park. When stopped you need to use the parking brake. The display shows your selected high gear and which gear it is in currently. I was surprised that the transmission worked with the exhaust brake by automatically downshifting to slow down. There is an "Eco" mode which changes the shift points.

The air brakes are something new to learn. Basically, the service brakes are fully locked by default when there is no air pressure. When the engine is started, air pressure is built up in both a primary and secondary system. When pressure is over some threshold, the brakes are no longer locked unless the parking brake is on. Pressing the pedal lowers the air pressure to the actuators which allows the brake pads to re-engage. Kind of backwards from what you expect. I.e. depressing the pedal bleeds air from the brake lines. The parking/emergency brake completely releases the air pressure which locks the brakes. There is an air-drying system as part of the air system to remove moisture and a filter for the air intake of the compressor (one or two?). Air pressure is also used for the suspension airbags but is fed from the secondary air system.

So when you are an a campground and the diesel pusher in the next space starts their engine at 6am and just sits there idling, there may be a reason. They can’t move until air pressure is built up and, according to the owners manual, even bring in their slides. The suspension needs to be up pressurized. They are not (necessarily) doing it just to annoy you.

There are two hydraulic systems on the chassis. An engine-driven hydraulic pump for the power steering, like most other vehicles, but it also powers the radiator fan. The radiator is located on the drivers' side facing sideways. According to what I've read, this is preferable to the rear-mounted radiators as it cools better and doesn't blow debris onto a towed vehicle. The downside is an additional mechanical piece with the hydraulic fan. A separate hydraulic system powers the slide mechanism, the leveling jacks, and the generator tray. I believe that this system uses an electric pump but I wasn't able to locate the pump or reservoir when we looked at the coach.

Something else that surprised me. The wheel lug-nut torque is 450 to 500 ft-lbs. I guess I'm not going to be rotating the tires. And that explains why there isn't a jack or a spare tire...

6 comments:

redlegsrides said...

Those are some specs! The brake system description hurt my brain a little bit.

Artie & Leinen's Grand Adventure said...

Will I still be allowed to drive it, even if I have no clue what you said above? BTW, your middle picture isn't showing.

RichardM said...

Air brakes do seem to work backwards from what I expected. I’m not sure how ABS is integrated in. I am glad to see that it has both ABS and front disc brakes. Many that we looked at had front drum brakes.

And decent engine specs but lower than the Cummins ISL. The Cummins engines are also common rail and not HEUI so they use very high pressure fuel instead of very high pressure engine oil within the injectors. A slightly simpler design. The HEUI system was developed by Cat and Navistar so it was also used by Ford.

RichardM said...

It probably would be a good idea to understand how air brakes work so you would know the significance of low air pressure. Or that the governed engine speed is the same as redline.

And, fixed the middle picture.

david R said...

Yeah, brake system explanation did not compute. Will have to read again or assume that it will work when called upon.

RichardM said...

It is backwards from what I had assumed. When there is no air pressure, the brakes are locked. Air pressure in the lines pull the pads and shoes back so the wheels can roll. Lowering the air pressure in the lines applies the brakes. This way, if there is an air leak or failure of the system i.e. no air pressure, the brakes are applied. Interesting design.