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BSFC and RPM's

BSFC and RPM's

I hope Catservice engineer reads this one.
A genset would be the best because of steady load to do a comparison. Older Cat diesel engines where rated at pretty low
rpms, since the low rpms represent lower air volume per unit time thus lower fuel flow. I would like to know how the economy of those older engines compare to the newer high rpm engines. Someone that has actual fuel consumption figures please chime in.

RE: BSFC and RPM's

I would have thought that a genset wouldn't give the sort of comparison that you'd want, as the revolutions are effectively fixed for generator applications.

EDMS Australia

RE: BSFC and RPM's

As FreddyNurk pointed out, the speed of a genset engine is a fixed ratio to the grid frequency. Small and medium 60hz gensets have been driven by either 1200rpm or 1800rpm engines, and very large ones by lower-speed engines (900rpm, etc).

While increased engine speed generally has a detrimental impact on fuel consumption, there are a number of other factors which tend to outweigh it:
- emissions tier (lower emissions usually seem to result in higher fuel consumption)
- turbo selection
- fuel injection technology
- probably some other big ones I'm not thinking of at the moment

There are exceptions to my statement about emissions - looking at Cat Tier 4 gensets for drilling as compared to Tier 2 ones, the fuel economy actually improved somewhat. They're both 1200rpm/60hz. The fuel injection system, stroke, and turbochargers were all changed in conjunction with the emissions improvement.

RE: BSFC and RPM's

BSFC should be significantly lower on the modern engines, youre comparing engines like the old fully mechanical D398 that ceased production 30 years ago with the latest electronically controlled D3500 which should be ~10% more efficient. If you want facts and figures dig through SISWEB, google, or call the local Cat dealer, they've published all this data for many decades.

RE: BSFC and RPM's

There is a lot of "it depends" in my answer on this. A lot of things have changed in diesel engines since I started working with them. When I started with a CAT dealer our workhorse engine was the 6.25 bore family, rated 1200 RPM for gensets and 1400 RPM for marine propulsion (but we also still had a bunch of 1000 RPM rated engines out too), average BSFC was about .355 lbs/bhp-hr. When a new piston and ring pack was introduced, along with a new turbo, BSFC dropped to about .345, but along with that change came less tolerance to extended time low load running. The 3500 mechanical engine came out in about 1983 for EPG, first units were 1200 RPM rated, what we called Phase zero iron, mechanical fuel injection (MUI) and big turbos. BSFC was better, but was way less tolerant to fuel variations, and a pretty steep learning curve came about. Phase 1 iron had many improvements, still mechanical injection, but at higher pressures and better spray angles with improved atomization. As I remember we were around the .330 mark at the time. The other thing that came into play was how flat the fuel consumption rate was over more of the load range, most of our turbocharged engines best fuel consumption was between 80-100%, below that and BSFC went up pretty quick. The 3500B introduced the first EUI (Electronic Unit Injected) fuel system, cam actuated but rate controlled by the ECM with the ADEM control, after that the ADEM evolved to a flash memory, big deal, now we could make changes and updates by plugging in a laptop and downloading a file.

When you try to use rated RPM to compare fuel rates it doesn't always work out so good, in the early 80's one of our biggest competitors for power in the US Tuna fleet out of San Diego was installing larger frame medium speed engines and beating us pretty darn good on fuel consumption and service intervals. Problem was they took up twice the room and weighed about three times as much.

Big changes in BSFC were driven by the on hiway trucking market at a time when CAT was trying to improve their share. The 3406B mechanical engine had the best BSFC in it class for a couple of years, mostly as a result of combustion chamber and fuel system improvements. About that time our last engine families with PC fuel systems were either replaced or upgraded to DI fuel systems, along with better turbo's and aftercooling.

The the first round of serious emissions reduction legislation hit, and BSFC took a pretty big jump, as the focus was NOx reduction, and increased fuel consumption, along with increased particulate and CO emissions came with the early efforts.

Then some really serious changes took place, ECM's changed dramatically, fuel system injection pressures increased by huge margins, everything inside the engine was looked at, pistons and valves went thru a number of changes, tighter tolerances,different combustion bowl shapes, various ring design changes, cam profiles, and a lot more. Air induction systems got pretty big revamps as well, from the outside turbo's didn't seem to look all that different, but internally a lot did change. Another big change, way more part numbers, now virtually every engine rating family had its own fuel injectors, turbo's and aftercoolers. BMEP went up, horsepower to weight ratios changed dramatically.

Then the EPA started the Tier levels, and pushing hard. Sometimes a new rating had lower emissions and higher fuel consumption, sometimes both were reduced, but in most cases it was a compromise between performance and exhaust emissions.

For a lot of years that magic target for everyone in the industry was to break .300 lbs/hp-hr, then CAT bought Mak and the big slow engines had beat that mark long ago. As I remember at that time their highest speed engine was 750.

So rated speed by itself isn't the main driver of BSFC in most of the engines I deal with. As ivymike mentioned above, when the first tier 4 drill engines came out there was an expectation by a lot of the end users that they were going to pay another fuel penalty to make the EPA happy, a lot of efforts went into making some of those latest engines do much better, and a lot of the technology is shared by other manufacturers as well, pistons are a good example of that, like the Mahle Ferro-therm, turbos are another.

The flip side of the coin is that the newer higher power lower emission and lower fuel consumption engines are also not as robust, don't like operating at low loads, are picky about what you feed them, when compared to some of the old iron.

I hope that answered your question, MikeL.

RE: BSFC and RPM's

I thank you Mike, good info.

(Added on 6-1-17)
The comparison I was looking for is, fuel use of a very slow speed engine to a faster speed engine of the same HP rating.
Since a PC system engine would be less efficient than a DI engine I guess my old Cat engine would not be the correct one to use.
I know BSFC, but still common sense says the low speed engine would be more economical due to the volume of air not used as compared to the higher speed engine. Then there is the longer burn duration and valve timing to accommodate the slower speeds. But if piston speeds are equal then I suppose this is all conjecture.

RE: BSFC and RPM's

Perhaps I am misunderstanding, but it seems the OPs question was not about GenSet characteristics but the effect of RMP of BSFC. Assuming I have the OPs intent correct, I offer this...

Though they are related it's easy to get confused when talking about BSFC versus efficiency. BSFC in the US is commonly stated in units of lb/hr/HP (where lb is pounds of fuel) and HP=RPM*Torque/5252 (see discussion at the bottom of I highly recommend this site to those learning about the details of engines. It's very informative and doesn't gloss over the details much). In contrast, efficiency is a simple measure of work out (Joules) versus energy in (Joules) and is much easier to follow IMHO.

Positive factors in the efficiency equation are compression ratio, burn rate and/or piston speed (more later), and expansion ratio. Negative factors include heat loss, pumping losses, mechanical losses, and friction. Compression ratio plays into the equation because work out is maximized by expansion and starting with a very small volume at the start of combustion naturally leads to a larger expansion ratio. Burn rate plays into the equation because we want the maximum period of expansion and, since the piston is moving down at combustion, some of the fuel burn occurs at volumes greater than the minimum with negative impact on expansion. You can see how piston speed plays into this equation; a slow burn rate and a fast piston speed means more combustion is occurring at larger volumes, so the expansion ratio suffers. Expansion ratio is obviously key to extracting work from combustion, but it is limited by friction; if expansion chamber pressure is less than that required to overcome friction, the engine is *expending* work to move the piston downward.

Note there is a paradox in dealing with burn rate and piston speed; A slow moving piston ensures combustion occurs when the chamber is closer to minimum volume, and that's good. On the other hand, a slow piston must contain pressure over a longer period and it loses some around the rings (blow-by). Furthermore, a slow piston implies the heat of combustion is exposed to cylinder walls and pistons longer, so more heat is lost. There is a way to work around this paradox by using Homogeneous Charge Compression Ignition (HCCI), but that's outside the scope of this discussion. See my comments under my "Create The Future" contest entry at for more information. Please register and vote for me while there !

Heat loss affects efficiency in the obvious way; engines are extracting work from heat, so losing heat is bad. Pumping losses are produced by the work required to draw intake charge into the cylinder and push exhaust back out. Mechanical losses result from the need to drive required equipment such as oil pumps and valve trains. Friction is an obvious detractor to efficiency.

Now lets look at spark ignition engines and diesel engines.

Spark ignition engines are typically controlled by limiting air into the cylinder (aka "throttling") and that creates large pumping losses. They then compress the charge as much as possible but are limited by knocking (aka pre-ignition) and thus can't attain very high compression ratios. Once the piston passes TDC, the spark fires and combustion begins (note most piston engines use high octane and/or spark advance to maximize compression before bulk combustion occurs, but that's a detail). Since the intake charge is well mixed, burn rate is determined almost exclusively by flame propagation speed which is pretty fast. In addition, the piston moves slowly near top dead center, so combustion occurs close to the minimum volume and expansion is maximized. Examining a plot of work out in a spark engine reveals an issue with crankshafts, however; they take a while before they really start producing torque, so they spend too much time containing combustion gasses before extracting work and suffer from blow-by and heat loss as a result.

Diesel engines are regulated by controlling the amount of fuel in the chamber and are not throttled, a plus for efficiency. They compress intake air before fuel is injected, and this allows for very high compression ratio. Once they pass TDC, they begin injecting fuel, and it ignites upon contact with the extremely hot air created by compression. This process is inefficient for two reasons. First, combustion occurs along the leading edge of the fuel spray where the charge is very rich and is neither complete nor clean. Second, the spray takes longer than flame propagation, so more combustion occurs at other than the minimum volume.

Now let's consider the impact of RPM on all this.

Engines running at high RPM are designed as air pumps such that gasses in the intake manifold have significant momentum, and this reduces pumping losses. On the downside, high RPM yields greater inertial loads on structures because the pistons are moving so fast. For this reason, high RPM engines typically employ less stroke in order to reduce inertial loads and compensate by using wider bores, but this increases combustion chamber surface area and heat loss. These high RPM engines still have good compression, but expansion is less because stroke is short and the exhaust valves have to open sooner. The reduced expansion ratio results in a lot of wasted heat/pressure in the exhaust. Note nearly all engines, whether spark or diesel, typically try to recover some of the lost energy resulting from less than complete expansion by using turbochargers (which reduce pumping loss and, if desired, apparent displacement).

The bottom line of all this is that diesels tend to be more efficient because the losses resulting from extended combustion are more than overcome by gains in compression and expansion ratio. The long strokes they use to gain increased compression/expansion, however, drives lower piston speeds to reduce inertial loads, and this increases heat loss. Diesels also tend to be quite heavy because their high compression ration results in higher combustion temperatures which drives a need for beefier blocks, heads, pistons, etc.

Which is better ? It depends on the application.

I hope this helps. Sorry if I rambled on too much !


RE: BSFC and RPM's

Nice lesson, I'm sure that engineers here learned lots.

RE: BSFC and RPM's

dicer, I apologize if that was too much or too simple for an engineering forum. I can never tell when some just wants an answer to a simple question or wants to learn, and I'm always worried I'm stating the obvious for professionals in the room. Sorry.

RE: BSFC and RPM's

All basic stuff, I learned that as a kid. And did not answer the question. Presenting DOD would more apply.

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