Mythbusting #1: “You need more fuel to spool that thing”

Many “conventional wisdom” clichés have grown up around diesel performance. Today, I’ll explain how a common one simply cannot be, and is not true: more fuel lowers EGT with some turbos.

First, the logical approach: fuel is potential energy. When we burn it, we convert potential energy to heat (thermal) energy. It follows then that the amount of heat is a function of the rate of fuel flow. So at a big picture level, more fuel= more heat.

Think in terms of energy balance to make sense of all this. When you increase the flow rate of fuel into the engine (for example, rev it up in neutral), the reason the engine is revving up is that it’s “rebalancing” to re-establish equilibrium with the higher level of energy input (more fuel).

When you rev the engine in neutral, you see very little increase in EGT, even near redline. Why? Because the engine is able to absorb almost all of the extra energy input by raising it’s rpm, there’s very little energy left to show up as hotter exhaust.

But what if I inhibit the engine’s response from occurring by putting additional load on the engine? As load on the engine increases, the engine absorbs less and less of that energy through its own response and that leaves more and more energy left over. This shows up as more heat into the coolant, and hotter exhaust.

Eventually, you get to the point where you have enough extra energy in the exhaust to surpass the minimum response threshold of the turbo.

Once the turbo is in its operating range, you create a situation where the turbo essentially feeds itself—or seems to. As you push the turbo farther and farther into its operating range, you get less and less in return. You simply run out of operating range. Once you’ve pushed that compressor to where it’s barely 60% efficient, and the CAC is not dumping as much heat as it was before, and all those negatives start to work against you, the engine simply cannot absorb all that energy any more.

The end result is that the energy shows up as hot EGT. Adding more fuel once you’re at the point only makes things worse. Continuing past this point will almost inevitably lead to engine failure.

Therefore, if someone is claiming to you that the reason your single turbo at 45psi has high EGT when towing is that you “need more fuel” you can assure yourself that this is a fool’s errand. Adding fuel under those conditions will only push your compressor farther past it’s operating range; it pushes the CAC farther past the heat amount it can shed; so on, so forth.

If you have high EGT when towing with your single, you need to 1) increase RPM, 2) reduce load (downshift) and 3) reduce fuel fed to the engine

JH

 

Good theory as always, but not real in practice.

Some turbos are not YET into their efficiency range at a specific fueling rate. Adding fuel in these cases cools the charge by driving the turbine and obviously, the compressor harder.

In this case, only gearing down, reducing load, ect will work.

The true test of any system for me is comparing two turbos at steady state. In other words, driving at 60MPH, 6% grade, 10K trailer. In this example, a larger charger would typically use more fuel. Why? since the boost threshold is lower, it takes more throttle to get the same combustion pressure.

And to further confuse it, try the same thing unloaded at 60MPH flat level ground at sea level. Now both chargers only need the same amount of fuel, although the boost levels will be different.

This is the issue with larger/ higher HP supporting chargers vs. smaller chargers.

In my own twin sets. I get higher EGTs with the larger set. If I add 5K trailer to the same grade as my old smaller set, I get reduced EGTs. How can that be? That flies in the face of logic! After all, I am obviously using much more fuel to move that load. And in reality, I am loading the chargers into the point they become most efficient.

So, while theory is great for discussion and for making general comparisons of two like situations, it cannot trump real experience doing it.

But going back to the guy that is towing with a single at 45psi and has high EGTs. By and large you are correct. The caveat is if that guy's charger is not into the point where it becomes self sustaining.

And adding fuel per se is not possible anyway. If you add fuel and change nothing else, you would increase speed. And that throws out the ability to add the fuel in the first place.

Too much typing this early in the morning. I am going fishing.

Dave

 

That's a good point, Dave. I really should have clarified better the specific case of an overboosted single that is already pegged.

IN YOUR case, with twins, it makes perfect sense to me that you'd have lower EGT when towing if the turbos are set up that way.

Load on the engine not only increase EGT, but it also greatly increases the velocity of the escaping exhaust. Faster exhaust has more energy and drives the turbo harder. That's why you can see boost threshold in RPM gets lower as you go taller in the gears. ON the hwy, I can see boost at 1100 rpm. In 2nd gear, I have to peg the tach to get even a start of real boost.

So yes, it's possible that more fuel WILL reduce EGT but only if that is the difference between being in or out of your turbo's operating range.

Far more effective, imo, though to downshift and use additional RPM to help drive the turbo harder at the same fuel rate..

Good catch by our fishin'guide!

 

Good stuff, how do you know when u are past the chargers sweet spot or if u are running under it? I am no expert, but if under load and you still can't get the charger into the self sustaining sweet spot and your egt's get hotter then you would need more fuel, right??? But if under load and u are past the sweet spot and egt's get hot you would need to decrease load, raise rpm, or defuel, or get a charger that will match your fueling under load, right???? Is that what you guys said??? Did I understand right???

 

If you are running under it, you'll have low boost and high EGT. If you are making more than 15psi or so, you are well into the turbo's operating range.

On the high end, it varies based on compressor map and turbine housing. Sometimes you'll hit the "choke point" of the turbo when your turbine housing becomes the main restriction. Other times, you'll hit it when you run off the compressor map on the right side, though it's far more likely you'll run out of PR (top of map) than run off the right side of the map with most turbos. The CTD will only rev so much.

This is why twins work so well-- they have a much larger combined operating range, so they can in effect be both bigger and smaller than a single turbo-- and you won't run out of PR capability, either!

 

If you are under the turbos sweet spot and you are billowing smoke, adding more fuel will hurt it more so. There is a point that adding more fuel will actually decrease your EGT's, but that point is fiercely rich. Usually reffered to as 'putting out the fire'. Its easy to test on a 12v. Take a truck with a tuned AFC and punch it, then gut the AFC and do that again, and see which one takes off harder and cleaner. Thats why rolling into the throttle works better than punching it. Thats the one thing I never liked about the Smarty.

 

Can anyone explain something to me. My 64/71/14 is making 58psi average, although I've seen spikes to 62psi already. Egts are well in check, but I've been told that I'm well out of efficiency range. I am going to back it off to around 50psi & see how it does, but what could cause this to happen? I surely can't be overfueling this turbo. Right?

 

Most likely it has an adjustable boost elbow/ It's probably restricting all flow.

The least amount of boost you need to run to get the coolest EGTs will result int eh highest HP. If you are pushing a turbo too hard, it'll just act as an engine brake.

Dave

 

The part I don't understand is that I never changed my turbine housing or adjusted my wastegate since the 62 was on there. I was seeing 56psi from the 62 & meant to adjust it, but never got around to it.

 

Case in point for HOHN:

I had a 64/13/71 on my '01 with 3.54 gears and a 5-speed. When I was looking at this turbo and asking DTR member opinions, HOHN came on and said I would have to run alot higher rpms to keep things in line. I disagreed and drove the truck empty at 1800rpms cruising and enjoyed it. Things were great and I had taken peace with myself in making the right choice in a turbo.

Then I hooked to a trailer......EGTs were near uncontrollable. I couldn't even get on the highway without seeing 1400* before shifts. Boost was at a reasonible 45psi, so I couldn't figure it out. Then I remembered what HOHN had said about rpms. On the next tow trip I never got out of 4th gear. I ran at or over 2500rpms the entire 250 mile trip. MPG's were horrible, but EGT's stayed in check.

And yes, many told me I needed more fuel to get, and keep it spooled.

 

If you are delivering the same turbine HP to a larger compressor, maybe you are getting more boost for that reason?

You guys realize the 50psi is over 4.4PR-- and VERY VERY few singles have a map the runs up that high.

Ths BW 66mm compressor map I've seen stops at 3.5PR basically.

A PR of 4:1 at 3000 rpm is roughly 70lb/min.

So if you're running a turbo that flows more than this, but doesn't have a PR that will extend past 4:1, there's little point to it. The problem is that it takes a lot of PR to for a small 5.9 to flow huge masses of air, even when spinning it fairly high.

TWINS TWINS TWINS if you want big power...

..or nitrous, maybe?

JH

 

Now, Hohn, I see you post here, and I have a heck of a respect for your knowledge. I think that this post is aimed at people that use 'more fuel' out of context.

I said that to say this...

When I first got my turbo, with the stock sticks, I had to manually shift to keep the EGT's manageable. I mean, it was stupid. 30 MPH, and I was at 900 degrees.

As soon as I got these 2.5's, I felt better than a stock truck. EGT's were awesome, turbo spooled like I was spraying.

It depends on the application. In my case, I had too much turbo, and not enough fuel to lite it on the bottom.

I do agree with what you are saying, tho. There has to be a good balance.

Just my .02, it's not worth much, just my humble opinion.

 

here is why a lot of people say add more fuel. 9 times out of 10 people put a turbo on that is too big for the fueling rate they have.

 

That was true for me at the time, but I had plans for bigger sticks, just had not got them in yet...

 

I think I need more air for my altitude. I feel like I am pushing my hybrid to it's limit. I finally weighed my truck/ new trailer combo fully loaded- 24,890. When I get a headwind my egt's will stay ~900 to 1100 deg, I will drive for hours like that. It will cool off headin down hill tho.