Hill Climb and Descent Control 2008 ?

Don't know how tongue-in-cheek that was, but the "engine brake prohibited" signs are most often talking about the proper compression brakes ("jake brakes") on large semi-trailer tractors. (And on the Dodge pickup trucks with the Cummins diesels, I guess.) Those engines have a special cam setting which opens the exhaust port at the top of the compression cycle; since the wheels push back through the transmission to compress the air in the cylinders, that allows for true compression braking.

Normal gas motors are actually vacuum braking more than anything else: the compression isn't let out, so it just expands again. do not know whether the Atkinson cycle changes that at all.

See also: Jake brake - Wikipedia, the free encyclopedia

 

Heh. The ones here (SW USA, mostly NM and AZ) are text-only, although the text varies: "engine brake prohibited", "engine brake muffler required", "jake brake use prohibited", etc.

You're right that the Prius ICE can sound dangerous at high RPMs, but the Wikipedia article accurately describes unmuffled Jake Brakes as sounding like a "machine gun". If nothing else, you've 4-5x the volume getting compressed and discharged (relatively instantly, from a high- (diesel-level!) compression), vs. the relatively gentle exhaust pressure level on the Prius.

(And I've been doing a fair bit of climbing at 75 mph lately, so I know what a screaming ICE sounds like!)

Seriously, you're not going to get pulled over for that particular infraction... thus my asking whether you were joking or not. If you do get pulled over, your "it was in 'D' not 'B' officer!" defense is actually a completely technically correct and valid one; large diesels traditionally have a separate control for whether the engine brake is active, and if you are not deliberately engaging the engine brake. then you should not be found in violation of the ordinance. (Whether they need to then rephrase their signs as "Prii not welcome" is another story.)

 

The "engine braking" you are talking about is almost silent from outside the car. The reason there is a picture of a truck on the sign is because it applies to trucks.

There is a massive difference between the noise of a Prius spinning the ICE against compression and a GM or Cummins diesel barking through a sports muffler or no muffler with the jake brake opperating and a cowboy at the wheel.

 

For those that don't understand the braking method of the Prius, it is very similar to that used on electric trains.
During normal braking, instead of the brakes applying and turning the car's momentum into burned out brake pads and heat, the Prius turns that energy into electricity. To protect the batteries when they become close to fully charged, that energy is partially sent to the motor/generator attached to the ICE, which causes the engine to rev up, and uses engine compression and friction to retard the vehicle speed. That is commonly referred to as Dynamic Braking. When the speed reduces to around 5 KM/H, the electric motor no longer runs fast enough to produce electricity, so the normal brakes blend in automatically to compensate for the loss of braking effort. When you place the gear lever into the 'brake' zone, you are using Dynamic Braking. It is inefficient, DON'T USE IT, unless you are going down very long steep grades.
With electric trains, electrical energy is forced into the overhead wires for use elsewhere within the network during braking. If for some reason the overhead wire can't take it, the train automatically diverts that electrical energy to resistor grids located on the train, and converts it into heat.

 

A point of clarification relating to the above post: Braking though the electric motors is called dynamic braking. When the generated electricity is saved for later use, it is usually called regenerative braking, although it is still a form of dynamic braking. The Prius does dynamic braking automatically; you don't have to do anything special to use it. The recovered electricity is stored in the HV battery until it reaches a high limit, then the excess is used to spin the ICE as an energy waster. Friction brakes are employed if the dynamic braking is insufficient, or at speeds lower than eight mph. Using B mode on the shifter tells the Prius to immediately start wasting energy by spinning the ICE. There are also some minor modifications to the regeneration curve, but otherwise the braking in B mode is the same, with the same shift to friction brakes as needed.

Tom

 

From my tests, the Prius actually has a "hill holder", it's just electronic. When in "D" or "B" the HSD will not allow the car to roll backwards (unless you overpower it). It -can- move a bit due to play in the gears.

I had a Subaru and its' "hill holder" was a ball bearing in the brake circuit which held the brakes until you released the clutch. The ball bearing was a "gravity detector", to tell when the car was stopped on an uphill.

As far as their claim of downhill control, it's probably "B" they are talking of. It uses engine compression to dump energy, and thus "control" the downhill coast.