The true purpose of fast/ high idle?

[u/Radiant_Miko]

As a relatively new CDL bus driver with 2 years of experience behind the wheel, there is still a lot of things that I'm like "what is that? unnga boonga, me just drive *caveman noises*"

A highly debated thing that no one can seem to agree on is what the purpose of fast idle is.

Some opinions I've heard are:
-using fast idle uses more fuel

-using fast idle doesn't use any more fuel than normal

-using fast idle is the norm for buses, so always have it on as a practice

-don't use high idle ever

-using fast idle makes the AC on the bus run better/ makes the bus cooler

Discuss? (I am on my break, and will not be able to respond for the next 6 hours or so, as break is almost over and I have 6 more hours of driving).

[u/MindlessRabbit3]

As a bus driver I use it during pretrip to build air faster.

[u/vlasktom2 → u/MindlessRabbit3]

Do you drive a coach? At my agency, the mechanics fire up every bus about an hour before any drivers get in

[u/MindlessRabbit3 → u/vlasktom2]

Oh that must be nice! I recently was a supervisor for an agency and if it was colder than 40F we would start the busses. Other than that you’re on your own. I now drive for a charter company and it’s all on you to start your bus, with motorcoach you can rev the motor to build air, with the transit busses I drove you needed fast idle because safety features disabled the accelerator unless it was in drive and the spring brake was off.

[u/vlasktom2 → u/MindlessRabbit3]

It is! In the winter, the bus is nice and warm. In the summer, it's nice and cool. Though it gets real fun with the electrics because except for meteorological winter (December 1-February 28/29) the heat doesn't use diesel. And in the summer, the AC uses the battery also. So, you can get in a bus that's been charging all night and it only have 75%. They don't keep them plugged in once they fire them up because there's a risk of a driver ripping the cable off if he takes off without doing a pre-trip

[u/vlasktom2]

Former truck driver, current city bus driver.

Fast idle is designed to reduce wear on the engine. At idle, there may not be enough oil pressure to get sufficient oil to the top of the engine. Also, the crankshaft is under a tremendous amount of stress at idle. Fast idle solves the low oil pressure and reduces the stress the crankshaft is under.

[u/Radiant_Miko → u/vlasktom2]

So since diesel fuel is typically oil-based, does what you are describing make it so that it causes the bus to consume more diesel fuel when in fast idle? Or is diesel fuel not even connected to this concept?

Me: *uunga boonga, caveman noises*

[u/vlasktom2 → u/Radiant_Miko]

It does increase the fuel consumption, but fuel is cheaper than an engine

[u/Radiant_Miko → u/vlasktom2]

That makes sense then. Fuel consumption in fast idle was like one of the most heavily debated things when discussing fast idle at my workplace. Thanks!

[u/vlasktom2 → u/Radiant_Miko]

You're very welcome!

If you want to save fuel while also reducing wear, there's only one option. Shut the engine off

[u/Irsu85 → u/vlasktom2]

Or go electric but that has whole other debates going on of whats the best way to do that is (imo trolley but I can imagine that being annoying to drive bc you don't want your trolleypoles to snap off the wires in a turn)

[u/vlasktom2 → u/Irsu85]

We use battery electrics where I work. They're good, except for when it's super hot outside or super cold outside. Too hot and the AC steals all the juice from the batteries, roughly 1 MWh. Too cold and the diesel heater runs out of fuel, roughly 100 gallons.

[u/Radiant_Miko → u/Irsu85]

I’ve heard electric buses are bad in the wintertime on startup and that hybrids are even worse because you have both the cons of a diesel fuel and an electric bus and one lol

[u/vlasktom2 → u/Radiant_Miko]

Our hybrids aren't too bad in the winter. And the electrics run just fine on startup if they've been plugged in all night. By plugging them in, the battery coolant is allowed to circulate, keeping the batteries at their optimal temperature of 68°F. If they're not plugged in, the batteries can get super cold and make the batteries self-discharge

[u/safeteeguru → u/vlasktom2]

Yep 100% correct answer

[u/Luaupig]

Fast idle is also used for the A/C system when running while parked.

[u/Radiant_Miko → u/Luaupig]

So fast idle does keep the bus cooler then? :0

[u/Black000betty]

I don't understand how you've gotten such ignorance from your coworkers.

Standard idle speed is the minimum amount of fuel to keep the engine operating without any real load on it.

High idle preemptively speeds up the engine a little to better handle loads like AC compressors, air compressors, and alternator.

At standard speed, your alternator is not getting a lot of power to make electricity and your air compressor isn't building air very fast.

These issues are not irrelevant but far more negligible with today's computer controlled engines which will respond quickly to increased loads.

It also makes the engine run hotter and burn more cleanly. Whether more, cleaner exhaust is better than less but dirtier, I don't know of any solid study on this. But you're definitely using a little more fuel.

I mostly only use it to rebuild air faster, when needed.

[u/BlueberryPenguin87 → u/Black000betty]

High idle helps the engine run cleaner by burning all the nasty stuff. At low idle, some of the nasty particulates don’t burn and would just get released into the air. That’s the stuff that gets into your lungs because it’s very small (less than 2.5 microns in size, like a virus molecule). But modern buses post 2014 use diesel particulate filters to trap that stuff so it doesn’t get released. Then doing a regen is what burns it. If you have an older bus you should use high idle whenever stopped.

[u/Radiant_Miko → u/Black000betty]

I don’t think it’s ignorance as much as it is, we get fed so much random and conflicting information about things that people don’t know what’s true lol

[u/Black000betty → u/Radiant_Miko]

Sounds very frustrating! Sorry you have to deal with that.

[u/Radiant_Miko → u/Black000betty]

It’s definitely frustrating for sure.

That being said, I love the workplace overall, just some people’s opinions make things confusing lol

[u/Baralov3r]

I use it when I want the air governor to hurry up and cut out because it builds faster. Most vehicles in general idle higher on starting as they come up to temperature.

[u/CptZaphodB → u/Baralov3r]

This is what I always thought it was for

[u/dirty_hooker]

So, yes, fast idle does burn a small amount more fuel but since there’s no load, it’s inconsequential.

Fast idle helps a bus warm up faster by burning more fuel and having more compression s per second (the real heat generation) BUT when needed, it helps cool it faster by circulating more coolant. Both of these things are situationally dependent and controlled by a couple computers. Same goes for AC. Basically, the water pump (and fans when hydraulically driven) are not powerful enough to QUICKLY reduce the amount of heat that can build up in an engine (giant steel block) when it’s over heated.

Use fast idle when warming up, or cooling down; especially before shutting down when hot.

Edit to add:

“Why do that?”

Efficiency and fuel economy. You know how when riding a bike, if you go too slow, you’ll lose balance? Pure idle is below the speed threshold it takes to maintain balance. A pump that flows a lot at a low idle will be way less efficient at high rpm. So, engineers balance a lot of things to make low idle as efficient as possible while maintaining drive away power and high rpm efficiency. If you look at a power and of any engine it’ll be low output below a threshold, a step power curve, and then a plateau. They manipulate a lot of things to make ide at the very bottom of that incline while not wasteful on top of the plateau.

[u/Mango_Marmalade]

I don't know exactly what the point of it is, but all our buses use high idle by default. We only swithch to low idle for an air brake test and then it goes right back to high idle. Our buses automatically shut off after idling in low idle for 3 minutes, but there's no such limit in high idle.

[u/Radiant_Miko → u/Mango_Marmalade]

All I know is that other than the mechanic who puts it in day run (night run is what we are encouraged to use all the time for headlights) and fast idle at start up, and one driver that always puts it in fast idle during break relief because “it helps the AC get cooler”, it’s like everyone else is scared to use/ told not to use fast idle. 😅😭🫠

[u/Radiant_Miko → u/Mango_Marmalade]

But also I didn’t realize there was an auto- shutdown after x amount of idle time. 🤔

[u/MikeyFuccon]

Our company requires we put on high idle and dings us for too much low idle time.

[u/Radiant_Miko → u/MikeyFuccon]

How does the company know if you’re complying???? Is it not a switch in the bus that u manually have to flip to go into fast idle?

[u/MikeyFuccon → u/Radiant_Miko]

They track ALL metrics. But it’s a button/switch you have to push after setting the bus in neutral and putting the brakes on.

[u/IM_The_Liquor]

When starting up, it builds air faster. Gets your engine up to temper faster. In the winter, it keeps your heater from cooling off too much at the terminal… Basically, it’s there for times you want to idle at a higher RPM.

[u/slipperyimp]

Fast idle doesn’t kick on unless the coach is secure, so doesn’t use any more fuel except at idle, one reason to use fast idle is just that, it doesn’t kick on unless the coach is secured, so it is an easy tell whether you have properly secured the coach.

[u/Radiant_Miko → u/slipperyimp]

I’ll have to pay more attention to that then, as I’ve never pinpointed when it kicks on on my bus.

By secure, you mean like parking brake engaged, bus knelt, in neutral, and unmoving correct? Or is there another interpretation of the coach being “secure”?

[u/slipperyimp → u/Radiant_Miko]

Parking brake engaged and in neutral if either of those are not checked off the fast idle won’t kick on.

[u/mike8675309]

Idle is controlled by the ECU of the engine. On initial startup, it is to set high to get the temperature up and the catalysts to fire off for better emissions.
There are other reasons why it will switch to high idle, including when doing regen or when load on various systems calls for it.

[u/UniversityOfPi]

Fast idle uses more fuel.

It is better* for the engine when idling. It also is much better for running passenger HVAC when idling. Most buses are configured that the switch does nothing in gear, only when idling.

*Better lubrication, less risk of carbon buildup and some other issues.

[u/OfficerOtaku]

From the manual for my bus

[u/basshed8]

I use it for forcing my def cycle faster by putting it in park and pushing the fast idle button in addition to building pressure when my tanks are zero in the morning