Cummins 4BT & Diesel Conversions Forums banner

1 - 14 of 14 Posts

·
Registered
Joined
·
115 Posts
Discussion Starter #1
Can someone explain to me why the 4BTA consumes more fuel at idle (~225 G/KW-H) versus 1800 rpm (~205G/KW-H)?

 

·
Registered
Joined
·
115 Posts
Discussion Starter #2
I think I see it now. If you multiply the Power (KW) and Fuel Consumption (G/GW-HR) you get G/Hr at each RPM. When you do that you see that you really use less fuel at idle (~4050 G/Hr) than at 1800RPM (~18450 G/Hr).

Right?
 

·
Registered
Joined
·
1,366 Posts
That's per HP. But I'm guessing that engines are designed to be more efficient at their operating range they'll be seeing most commonly, not at idle.
 

·
Super Moderator
Joined
·
8,728 Posts
That curve (BSFC) is essentially the engines efficiency curve, lower numbers are better. BSFC maps are around for some engines which give the efficiency over the whole operating range, but the full-load curves given are still a good tool.
 

·
Registered
Joined
·
330 Posts
I remember the surprise when I got straightened out on this issue. I was into airplanes and airplane engines and was surprised that horsepower and rpm are only indirectly related. The engine in question as most engines, I think, are rated at about 75% of max power. Which means, it can dissipate its internal losses continuously without doing damage to itself at that speed. What twisted my head around was when the discussion got around to what happens as you take the engine up to higher altitudes and the air gets thinner and the power drops off, what do you do to keep the power level up. Well, you increase the rpm. And, within reason, that's OK as long as you don't exceed the
75% of maximum rated HP, without regard to the rpm. So, while at sea level, 2250 rpm is a normal 75% of power, at altitude, it may take 3000 rpm to get back to 75% power.
As others have already stated, its the energy consumed at any given rpm that determines the fuel consumption and not the rpm per se.
 

·
Registered
Joined
·
7 Posts
I have seen several of these PDFs over the years, but none of them have partial throttle fuel consumption data. Is there a ratio or an equation to calculate that or a source to get the PDF that contains it? CPL 1963 or 1839.

Thanks
 

·
Registered
Joined
·
11,219 Posts
Not really sure what you are looking for. CPL 1963 and CPL 2264 were the 2 industrial models with the P3000 injection pump. CPL 1839 was the only road model with the P7100 injection pump. Cummins may be the only place to find a fuel consumption curve. Those 2 CPL's are similar but very different. Member CrewCab59 may have access to the fuel consumption charts by CPL. He is a Cummins dealer. If you need an engine serial # for those CPL's, 46779089 is a 1963 and 60124112 is an 1839.
 

·
Registered
Joined
·
5 Posts
I remember the surprise when I got straightened out on this issue. I was into airplanes and airplane engines and was surprised that horsepower and rpm are only indirectly related. The engine in question as most engines, I think, are rated at about 75% of max power. Which means, it can dissipate its internal losses continuously without doing damage to itself at that speed. What twisted my head around was when the discussion got around to what happens as you take the engine up to higher altitudes and the air gets thinner and the power drops off, what do you do to keep the power level up. Well, you increase the rpm. And, within reason, that's OK as long as you don't exceed the
75% of maximum rated HP, without regard to the rpm. So, while at sea level, 2250 rpm is a normal 75% of power, at altitude, it may take 3000 rpm to get back to 75% power.
As others have already stated, its the energy consumed at any given rpm that determines the fuel consumption and not the rpm per se.

Ugene knows his stuff!
A note for general guidelines has to do with what standards are accepted for horsepower and torque ratings. Manufacturers will design an engine to do it's best in specific ranges.(the sweet spot) For diesels it will be at sea level and 1850rpm. For gasoline it's sea level and 3600rpm. If you keep the sweet spot in mind when driving, you will do better.
 

·
Super Moderator
Joined
·
8,728 Posts
Ugene knows his stuff!
A note for general guidelines has to do with what standards are accepted for horsepower and torque ratings. Manufacturers will design an engine to do it's best in specific ranges.(the sweet spot) For diesels it will be at sea level and 1850rpm. For gasoline it's sea level and 3600rpm. If you keep the sweet spot in mind when driving, you will do better.
Keep in mind the 1800 and 3600rpm numbers have their root in generator speeds. That's for the 60Hz power you run in NA. Most of the rest of the world runs on 50Hz so our generator engines have to run 1500 or 3000rpm. Industrial pumps tend to run about the same.

In engine design your most efficient speed is determined by piston speed. Which is why bigger engines (with longer stroke) are more efficient at lower rpm and want to produce max torque at lower rpm. Too fast and the engine tries to out-run the pressure wave and your injection window gets too short. Too slow and it's hard to stay running smooth.
 

·
Registered
Joined
·
5 Posts
Cool. I never new where those numbers originated. Always seemed like something that somebody just pulled out of their ...butt one day.
 
1 - 14 of 14 Posts
Top