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Enginepowercurves.Pdf Dave Gerr, CEng FRINA, Naval Architect www.gerrmarine.com Understanding Engine Performance and Engine Performance Curves, and Fuel Tankage and Range Calcuations By Dave Gerr, CEng FRINA © 2008 & 2016 Dave Gerr eep in the bilge of the boat you’re designing, building, Between them, pretty much everything you need to know D surveying, repairing, or operating is her beating heart— about this engine’s performance is spelled out. her engine. The recipient of endless tuning, cleaning, and fuss, it’s the boat’s engine that drives her from anchorage to Maximum Output Power - BHP anchorage. Engines, however, come in a wide array of sizes, The maximum output power curve is just what it says. It shapes, and flavors. Whether you’re repowering, determin- shows the maximum power that the engine can produce (in ing which propulsion-package option to install in a new boat, ideal conditions) at any given RPM. This is also called “brake trying to optimize perform- horsepower” or BHP be- ance on an existing boat, or cause in the old days it to understand why an en- was measured on a gizmo gine isn’t achieving full termed a “Prony brake”— a rated RPM, good informa- form of dynamometer. tion on engine behavior can These days other types of seem hard to come by. The dynos are used, but the key to deciphering engine result is the same. Note performance is the perform- that the brake horsepower ance curves that are in- is maximum in every re- cluded with the engine gard—tested on a bench in manufacturer’s literature. a shop and before the re- We’ll examine these curves duction gear. Real power in here. the hot humid bilge of a boat may be somewhat Let’s take a look at a fairly lower. For our 420-HP CX typical high-output diesel, Yanmar, the maximum the Yanmar 6CX(M)-ETE, rated BHP is 420 HP, at and see what her curves will 2,700 RPM. The units for tell us. A pair of these might power on the graph are—as drive a 35-foot sportfisher- you’d expect—horsepower man; or a single engine on the right and the metric could propel a 65-foot mo- equivalent on the left, kilo- torcruiser. The info’ sheets watts. for this engine are handy because they happen to Output Power With Marine have all the usual curves. Gear - SHP Some manufacturers don’t Of course, almost all en- include the torque curve or, gines engine have a re- sometimes, the fuel- verse/reduction gear consumption curve. At any Yanmar 6CX(M)-ETE Performance Curves mounted on their tails. The rate, there are five standard reduction gear not only performance curves: allows the boat to back up (which I’m told is useful), but it’s what allows you, the designer, to match the torque charac- 1) Maximum output power without reduction gear teristics of the engine to the optimum propeller. All this is 2) Maximum output power after marine reduction gear both proper and also unavoidable. Like all other machines, 3) Propeller power curve however (and the reduction gear is nothing more than a ma- 4) Torque curve chine with lots of moving parts) reduction gears have built in 5) Specific fuel consumption power losses due to friction. Standard marine gears fritter Gerr Marine, Inc. | 838 West End Ave., Suite BB | New York, NY 10025 | t. 212-864-7030 | [email protected] Dave Gerr, CEng FRINA, Naval Architect www.gerrmarine.com Page 2 Engine Power Curves continued away about 3 percent of power as they do their required job. gine warranties require that the propeller allow the engine This means that the maximum “output power with marine to spin up to maximum RPMs or nearly so, otherwise any gear” or the SHP is reduced to 407.4 HP [420 HP x 0.97 = engine damage is likely to be blamed on overloading the 407.4 HP.] This is sometimes called “shaft horsepower” or engine and the warranty considered void. SHP. It is what the curve just below the maximum output power curve is showing—the maximum power at each RPM, Conversely, if the propeller had too little diameter or pitch minus 3 percent. the propeller power curve would flatten and extend out be- In reality, this isn’t true shaft horsepower. There is still an- yond and/or below the engine power curve (Propeller Power other source of loss due to friction—the shaft bearings. Gen- Curve B). In this case, the engine would spin up to max RPM erally, you lose about 0.5 percent of power for each bearing, with ease, but the prop would be too small to do useful work so—with one or two shaft bearings—the true power the pro- and, again, wouldn’t drive the boat effectively. peller sees is about 96 percent of brake horsepower or 403 HP for this engine. (If there’s a remote V-drive, subtract an- The goal is to have the propeller sized and selected so its other 2 percent of power for it.) maximum power demand exactly matches the maximum power (shaft horsepower) produced by the engine and maxi- Propeller Power Curve mum rated RPM. Because the curves are such different So far so good, but shape, they can’t the propeller power meet at any other curve makes things point, so this is a more entertaining. compromise, but What is it showing? the only one possi- Well, remember ble, and it’s one that the BHP curve that works well. is generated by testing the engine The way to make in a lab to get the the power curves maximum power match up more that the engine can closely at other deliver at each RPMs is to use a RPM. The word controllable-pitch “can” is crucial. The propeller. This is fact is that the quite useful for power the boat’s vessels that oper- propeller demands ate under varying or absorbs in- loads or run for creases or changes long periods at at a very different different speeds, rate relative to Propeller Power Curve Variations but the extra ex- RPMs than does pense is not called the output power that your engine can deliver. What the pro- for on most average boats. peller power curve shows is approximately the power that a The Missing Power Mystery standard propeller would be using at any given engine RPM. Okay, you may ask, but what about all that extra power that You can see just how different the shape between the two the engine is producing? If you look at the 420 CX Yanmar curves is—between the BHP (or SHP) curve and the propeller curves, it indicates that propeller power at 2,300 RPM is curve. 250, but that the engine is putting out about 380 HP at that same speed. What happened to the missing 130 horse- This is unfortunate in a way as it controls a lot of things power? The answer is that the engine isn’t generating it. A about propeller selection. If you installed a propeller that diesel engine’s power at any RPM is controlled by how much was too large in diameter or that had too much pitch, then fuel is metered into the injectors. This engine could produce the propeller curve (Propeller Power Curve A) would be 380 HP at 2,300 RPM, but since the propeller is only ab- shorter and steeper and would intersect the engine power sorbing 250 HP, less fuel is being injected into the cylinders curve at some point less than maximum 2,700 RPM, per- and less fuel means less power—even at the same RPMs. haps at 2,100 RPM. In this case, the propeller would be overloading the engine and lugging it down. This would limit Of course, if you added an auxiliary load (perhaps a high- speed, and would be bad for the engine. In fact, most en- output alternator) then this could add another 15 HP of load Gerr Marine, Inc. | 838 West End Ave., Suite BB | New York, NY 10025 | t. 212-864-7030 | [email protected] Dave Gerr, CEng FRINA, Naval Architect www.gerrmarine.com Page 3 Engine Power Curves continued above the propeller load. In this case, more fuel would auto- Specific Fuel Consumption matically be injected to keep engine at our throttle-set The specific fuel consumption curve reads—as is often the 2,300 RPM and add an extra 15 HP of output power. You case—in rather inconvenient units. In this instance, in grams could keep on adding loads at this RPM until you reached per horsepower per hour (g/hp·h) and in grams per kilowatt the maximum rated 380 HP, at which time the engine per hour (g/kW). For the moment, though, what we’re really would be overloaded, and you’d have to increase RPMs. interested in is the shape of the curve and where fuel con- Eventually, as you increase RPMs the propeller curve gradu- sumption is lowest for the output power and torque. In other ally rises until it crosses the engine power curve and there’s words—just the opposite of the power and torque curves— no more extra power available. the best spot on the specific fuel consumption curve is where it’s lowest. For the 420 CX Yanmar, this is at 2,000 In fact, the extra power we’ve been discussing here is a mi- RPM. nor consideration. The reality is that the two upper power curves aren’t what your propeller sees or uses.
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