Me and my friend have a question about how the engine break works?
Posted by ReinkDesigns@reddit | Truckers | View on Reddit | 10 comments
I was going down 40 NC>TN yesterday going up the hills to get there I was getting a out 5-6mpg. When I got to 40 I put it to 35 and set the engine break by the time I was at the bottom of the hill I was at 12+mpg. My question was how am I getting such good MPG when I'm basically at 1600-2000 rpm for 20 miles going down hill, but at 5-6mpg sitting at 1000-1200 uphill.
When I looked it up I saw a few things saying that the engine break cuts fuel to the engine, which made since as to how my fuel economy was so good but my friend said that's not possible because the pistons have to still be getting fuel to fire, and if you cut fuel the engine would shut off and even if it didn't there would be no exhaust to spin up the turbo.
So how exactly does the engine break work without turning the engine off or are we both missing something
OrganizationNo6167@reddit
Newer truck engine brakes actually kill certain cylinders, higher the level more cylinders are cut off. Less cylinders pumping less fuel consumed, also going down hill requires no power from the engine that’s prob why ur fuel averaged was good. But 20 miles will make no difference in terms of savings. The difference is like 1mpg
ReinkDesigns@reddit (OP)
O sorry I drive a Peterbilt and the MPG readout only accounts for the last hour of driving, I have to actually stop the truck and go into its settings to see trip data to see the overall mpg for the trip. It's really annoying that I can't just see my overall mpg at all times.
Antique_One7110@reddit
BRAKE v BREAK
There is a big difference between engine break and engine brake…although both will stop the truck.
18WheelerHustle@reddit
The truck's fuel injectors stop adding fuel to the engine. The engine's moving parts still create resistance, which helps slow the truck down.
InvestigatorBroad114@reddit
Diesel engines are compression motors not internal combustion, so they compress gasses to ignite where a gas engine needs a spark plug to ignite. And yes the injectors do cut out when you let off the throttle
ReinkDesigns@reddit (OP)
So this might be a dumb question then, how does the truck idle? Is definitely uses fuel while idling, so how does that work?
Smprider112@reddit
It uses fuel while idling, when engine breaking, the ECU cuts fuel to the injectors. The engine stays “running” because it is spinning, as soon as you apply throttle, fuel is resumed to injectors. If you slowed to a near crawl, without hitting brakes, the engine will resume fuel delivery to keep the engine running.
Pitiful-MobileGamer@reddit
What an engine brake does, is used the resistance of compression of air in the cylinders, to put stopping force in the driveline.
A normal diesel cycle, has the injector putting diesel in the cylinder right as the air compresses and reaches high temperatures. The engine brake doesn't do that, but it releases that built up pressure the top of the stroke, allowing the valves to refill and recompress. Without the power down stroke, the engine acts like a giant compressor, you're not burning any fuel because gravity is doing the work.
Waisted-Desert@reddit
If you turn your engine completely off your motor is still going to rotate at 1600-2000rpm going downhill. That has zero fuel consumption.
https://www.youtube.com/watch?v=HkfjCJClWVA
https://www.uti.edu/blog/diesel/how-diesel-exhaust-brake-systems-work
802trucker@reddit
The wheels turn the engine as long as you’re in gear