Tweaking Big Green

[Just Plain Jeff]

Since about 2000, Detroit and Prevost have been at work trying to reduce the exhaust emissions from Big Green:



Unfortunately, the result has been that the 475hp of the engine has been reduced to about 400hp at the wheels. And on an H3-45, for example, heat dissipation requirements have also increased and so has mileage.

The fix is to re-set the injectors to 75 and open up the pump outlet restricter for increased power and economy. This will result in about .7 mpg increase and some more power.

The future is a tad more grim: Within the next 18 months, Prevost shells will be fitted with a roof-type exhaust a la Blue Bird, as exhaust temp will reach 650F due to emissions requirements (good for melting asphalt, bad to stand near).

After that is the new Volvo 12.7 engine with the stack, to be made in Hagerstown, PA, the old Mack plant, which Volvo owns.

The good news is that if you have a pre-2000 coach, you don't have to worry about any of this, just have fun with what you have, as those days are gonzo!

[Jon Wehrenberg]

The pilots on this forum will appreciate the following.

On engines the maximum power at a given fuel flow is about 40 degrees F rich of peak (ROP) exhaust gas temperatures. For years the practice in general aviation planes has been to fly with the fuel flows set to 100 to 150 degrees rich of peak, a setting that is less harmful to the engine because it reduces internal cylinder pressures. (Nobody actually knew that at the time, but by following the practice the results were good.)

With the proliferation of engine analyzers for aircraft, the pilot can now see the actual exhaust and cylinder temperature on each cylinder individually. To that some pilots have added fuel injectors which balance the fuel flow so each cylinder reaches its peak temperature at the same time. When the plane is so equipped the pilot can adjust the fuel flow to about 50 degrees lean of peak (LOP).

All of a sudden there are serious changes. First, the engine literally burns every drop of fuel so the exhaust is very clean. That sounds like what the EPA is trying to achieve. There is more good news. The cylinders have less internal pressures as a result and it is presumably much easier on the engine. The fuel flows are also lower. But the downsides sound like those on big green. As the engine is run LOP, power drops off, and the exhaust gas temperature climbs significantly. The limiting factor when running LOP is the temperature of the exhaust gases entering the turbocharger.

If this is the direction Detroit is heading, there are going to be unintended consequences. The engine will be extremely sensitive to fuel flow and a slight change will cause it to run leaner, causing a lean misfire, or slightly richer, driving exhaust temperatures even higher and subjecting the engine to potentially damaging operating conditions.

The worst part of this whole situation is the concepts regarding internal cylinder pressures and exhaust gas temperatures may not be understood by mechanics (they aren't in aviation as a rule) so repair work will tend to focus on the symptoms and not the cure unless the tech writers who do the repair manuals are successful at teaching mechanics how to properly diagnose things. Having DDEC may be crucial.

[lewpopp]

NO $HIT JON !!!

You don't realize how long I've been waiting for that post. I was just on the edge of surviving until that came along so now I'm good to go. Where, I don't know.

[lewpopp]

NO $HIT JON !!!

You don't realize how long I've been waiting for that post. I was just on the edge of surviving until that came along so now I'm good to go. Where, I don't know.

do you engineers fly in the nude so you can admire yourselves all over?

[lewpopp]

They finished working on ,or leaning on, my coach this afternoon. Took it for a run and it seems to be ok.

I really won't be able to tell for sure until I get a few more miles under my belt.

Haven't been presented with the bill yet.

I noticed that the 6 rear airbags are looking 16 years old. Where would you have them installed and where would you purchase them?

[lewpopp]

I'm wondering....

Should I grab the slightly bent radial arm or leave it there? Should have asked this question earlier cuz I'm leaving in the am.

[Jon Wehrenberg]

Lew,

I have no idea what Prevost suggests for airbag replacement, but I can tell you my experience. On my 1987 I started to experience microscopic pin holes in the bases due to corrosion from driving up in the snow belt. The rubber portion of the bags was OK. They were 10 years old.

For my personal tolerance level, I am going to change all my airbags next year just because they will be 10 years old and an on-road airbag failure makes it impossible to drive.

There are undoubtedly much older coaches running around with the original airbags, so I may be too cautious. If you change them, expect a competent shop with the right equipment to be able to change out the rear bags in about one hour to one and one half hours each. That is the time it took me to change mine on a barn floor.

A bent radius rod is not a good thing, but if your coach is in alignment and Prevost does not think safety is compromised I could be convinced to leave it. It depends on if it is a serious bend or if it is a slight bend.

[truk4u]

Back in the days of Corsair's, one ace pilot always seemed to have enough fuel to get back from missions, while the rest ran out of fuel, revealed his secret. Lean it out and under square the MP/Prop settings. Fortunately, uncle sam was buying the engines, the high pressure and heat toasted the engines.

I know you love this Lew, but heat is the killer in engines. Glad I have an old green one.

[Just Plain Jeff]

So, where's the Manual Override switch in the birdie engine control system?

[lewpopp]

Jon

Thanks for the reply. I will wait a while before I do the rear bags. I have to shake the brains and smoke loose from my mind after this last trip to Prevost.

I will post very soon what the outcome was. I am on the road on my way to Myrtle Beach and need some time. I'm hurting real bad.