Rolls-Royce Merlin/V1650 Engine

Key Specifications (Prior to racing modifications) 

Note: Rolls-Royce and Packard built Merlins became available after WWII; their first application in Unlimited Hydroplanes occurred in the early '50s. The Packard V-1650-9 quickly became the most desirable, with the improvements of the 100 series incorporated in the engine and an efficient 2 stage supercharger with the Bendix Stomberg Injection Carburetor and an intake elbow which could be modified to use in the downdraft position desired for boats.

Introduction

The Rolls-Royce Merlin engine is recognized at the powerplant that won the battle of Britain during WWII. The Merlin powered Hurricane and Spitfire were instrumental in neutralizing the German Luftwaffe's attempt to gain air superiority over Britain, reducing the effectiveness of German bombing and preventing an Axis invasion. The North American P-51 Mustang became an outstanding fighter capable of long-range bomber escort when the Merlin was adopted as the standard powerplant. One of the significant differences between the Allison V-1710 and the Rolls-Royce Merlin was the Allison relied upon a GE turbocharger to maintain high power at altitude, while the Merlin used two speed (and eventually two stage) supercharging. GE was unable to produce the turbochargers in sufficient quantity to equip both bombers and fighters, so aside from the P-38 Lighting which was equipped with turbochargers, most Allison-equipped planes were limited to relatively low altitude operation -- under 20,000 ft. When powered by the Merlin, the Mustang was able to achieve excellent performance at altitudes above 30,000 ft which allowed it to effectively combat Axis aircraft while performing high altitude bomber duties.

Boat racers began experimenting with Merlin engines in the late 1940's, but it was the Allison engine which became the established Unlimited powerplant in the late 1940's and early 1950's, the Golden Gate III first using an Allison in 1946 and the Miss Peps V winning the 1947 Gold Cup with Allison power. It was the Slo-mo-shun team which first successfully used the Merlin engine, becoming the first team to win the Gold Cup with Merlin power when in 1954 the Slo-mo-shun V won the Seattle Gold Cup race. Allisons would capture the next couple of Gold Cups, but for the next 30 years Rolls-Royce power would capture most Gold Cups and almost all the National Championships. From 1960-1979, the Merlin won 19 of 20 National Championships, and 17 of 20 Gold Cups, Allisons taking the others. In the first half of the 1980's, Rolls-Royce Merlins and Griffons split Championships and Gold Cups, with the Merlin and Griffon each capturing two Gold Cups each, and the Griffon taking three National Championships to the Merlin's two.

The Merlin was generally more temperamental and less reliable than the Allison, but it did produce more power. The better financed teams generally used Merlins, while those with less money typically used Allisons. While it can be argued that the Merlin was more successful than the Allison for various reasons, it is important to recognize a significant reason it was successful was because the top teams adopted it, making it the defacto standard among the leading teams. The mere fact that the top teams were using Merlins was an important factor in its success.

Merlin-powered Hydroplanes

Boat racers began experimenting with Merlin engines in the late 1940's, but it was the Allison engine which became the established Unlimited powerplant in the late 1940's and early 1950's, the Golden Gate III first using an Allison in 1946 and the Miss Peps V winning the 1947 Gold Cup with Allison power. It was the Slo-mo-shun team which first successfully used the Merlin engine, becoming the first team to win the Gold Cup with Merlin power when in 1954 the Slo-mo-shun V won the Seattle Gold Cup race. Allisons would capture the next couple of Gold Cups, but for the next 30 years Rolls-Royce power would capture most Gold Cups and almost all the National Championships. From 1960-1979, the Merlin won 19 of 20 National Championships, and 17 of 20 Gold Cups, Allisons taking the others. In the first half of the 1980's, Rolls-Royce Merlins and Griffons split Championships and Gold Cups, with the Merlin and Griffon each capturing two Gold Cups each, and the Griffon taking three National Championships to the Merlin's two.

The Merlin was generally more temperamental and less reliable than the Allison, but it did produce more power. The better financed teams generally used Merlins, while those with less money typically used Allisons. While it can be argued that the Merlin was more successful than the Allison for various reasons, it is important to recognize a significant reason it was successful was because the top teams adopted it, making it the defacto standard among the leading teams. The mere fact that the top teams were using Merlins was an important factor in its success.

Some of the most significant Merlin-powered Hydroplanes include Slo-mo-shun V (from 1954 on, 1954 Gold Cup winner, first for Merlin power), Miss Thriftway (converted in 1957, 1957 Gold Cup winner), Hawaii Kai III (1958 Gold Cup and National Champion, first Merlin powered National Championship), Miss Thriftway/Miss Century 21 (Gold Cup 1961-1962, National Champion 1960-1962), Miss Bardahl (Gold Cup and National Champion 1963-1965, 1967-1968), Miss Budweiser (Gold Cup 1969-1970 and 1973, National Champion 1969-1972, 1977), Atlas Van Lines (Gold Cup 1972, 1977-1979, 1982-1984, National Champion 1972, 1976, 1978-1979, 1982-1983), Pay 'n Pak (Gold Cup 1974-1975, National Champion 1973-1975).

Merlin/V-1650 Description

The Merlin is a conventional overhead cam liquid cooled Vee-type engine with 4-valve pentagon roof combustion chambers using two 6-cylinder monoblocks bolted to a split crankcase. The engine has a propeller reduction gear on one end of the engine a supercharger on the other. Cylinders are numbered from the propeller end, with the bank to the right when viewed from the supercharger end called the "A" bank (the oil pressure regulator is mounted to the outside of this bank for reference) and the left the "B" bank. The "A" side magneto fires the intake spark plugs, and the "B" side magneto fires the exhaust plugs. Unlike automotive engines, the cylinders were numbered 1-A to 6-A on the right and 1-B to 6-BR on the right, 1-A being the "rear" cylinder next to the propeller and on the left side when viewed from the supercharger end.

Type: 12 cylinder 60° Vee liquid cooled

Cylinders: Bore 5.4 in, Stroke 6 in, Displacement 1,649 cubic inches (27 liters). Compression ratio 6.0:1. Two cylinder blocks of six cylinders each comprising a cast aluminum-alloy cylinder head, six hardened steel cylinder barrels and a cast aluminum-alloy cooling jacket. Barrels held in head by a clamping action between head and cooling jacket. Barrel sealed by means of head gasket, rubber seals, and rubber triple seals and sealing rings. Cylinder head is fastened to block with 24 short studs; in addition 14 long, high-expansion steel studs clamp head/cylinder block assembly to crankcase. A number of transfer tubes/seals are used to seal mounting bolts and provide coolant passages between the head and the block. Combustion chamber has two intake and two exhaust valves and two diametrically opposed park plugs. Cast iron intake valve guides and phosphor bronze exhaust guides are used, along with alloy-steel valve seats.

Pistons: Machined from aluminum-alloy forgings with an elliptically ground outer diameter. Three compression rings above piston pin -- one keystone ring in the top groove and two conventional rings, and two oil-control rings in a single groove below. For racing, the ring package is often modified with a Dykes or head-land ring at the top, followed by one compression ring and a pair of oil-control rings with a spacer, the oil control rings occupying a single ring slot above the piston pin, the oil control slot being drilled radially to facilitate collection of oil scrapped by the rings. A floating piston pin is retained by snap-rings at each end.

Connecting Rods: Steel I-beam connecting rods are of the fork and blade type. The forked rods operate on the "B" bank, and the blade rods on the "A" bank. All rods have separate bearing shells. The forked rod is of the "marine type" comprising an upper strut bolted to a split steel bearing block assembly in which is carried the removable-type silver-lead indium lined, steel-backed bearing shells that bear on the crankpin. The blade rod low end is fitted with a steel-backed bearing shell lined with a silver alloy, which bear directly on the forked rod bearing block.

Crankshaft: The crankshaft is forged of nickel-chromium molybdenum steel, machined, nitrided, and statically and dynamically balanced. It has seven bearings and six throws with the crankpins numbered consecutively 1 to 6 from the propeller end. Eight crank webs extend beyond the journals opposite their respective crankpins for counterbalancing (racers have found it is necessary to weld additional weight to the crankpins for reliable operation at high RPM). The seven journal bearings are precision removable steel shell bearings comprising 14 flanged steel half-shells (two halves per journal) with a thin layer of lead-indium plated over a bearing silver foundation. Thrust is taken by the center main bearing. The end bearings are not grooved, the center bearing has two partial annular grooves, and the 2, 3, 5, and 6 bearings have 360 degree grooves. A dowel keeps each journal bearing from rotating. The crankshaft is drilled to permit oil passage, the journals are sealed by end plugs secured with through bolts.

Crankcase: Two aluminum castings split on horizontal centerline. The upper crankcase has two engine mounting feet and carries the crankshaft and mean bearings, cylinder blocks, accessories, wheelcase, supercharger, and reduction gear. The lower ends of the cylinder liners fit into openings in the upper crankcase and the cylinder blocks seat on inclined face. The cylinder blocks are bolted to the upper crankcase with 14 high-expansion steel studs each. The crankshaft is retained by forged aluminum main bearing caps which are seated in milled cutaways in the upper crankcase. Each main caps is retained by two large studs (one of which serves as a locating dowel) extending from the main bearing web in the crankcase and are also transversally cross-bolted through the sides of the crankcase. The upper crankcase also has an oil pressure relief valve located on the "A" bank and a generator-drive and support on the "B" side.

The lower crankcase serves as an oil sump, and also carries an oil pressure pump and three scavenge pumps, with provisions for an optional hydraulic pump. Baffles in he lower crankcase help reduce oil surge and improve oil control, particularly at unusual attitudes which can occur in flight. Each of the two primary scavenge pumps is equipped with removable oil screens; one scavenge pump collects oil via an extension pipe to a pickup at the propeller end of the engine, the other scavenges oil from the sump at the supercharger end of the engine. The third scavenge pump is the auxiliary scavenge pump used to collect oil from the supercharger impeller bearings -- it is located directly below the pressure pump.

Large studs on the face of the upper half pass through main bearing webs on lower-half to clamp the two halves over the bearing shells. Center main bearing provided with faced flanges which bear upon the center crank cheeks to provide axial location (and absorb thrust loads) for the crankshaft. Cast magnesium-alloy oil pan bolts to the bottom of the crankcase lower half. Oil is scavenged from front and rear or the oil pan.

Valve Gear: Two intake and two exhaust valves are used per cylinder. Both valves are forged from chromium-nickel-tungsten steel, but the intake valves have their seating faces protected by a nickel-chromium coating (Bright-Ray), while the exhaust vales have hollow/sodium-cooled valve stems and the entire valve head is protected with a Bright-Ray coating. The valve stem have very hard tips applied, and have a circlip to prevent the valve from accidentally falling into a cylinder if a spring breaks. Each valve features dual coil springs (an inner and outer spring) which are retained by a conventional collar and a split-cone keeper.

A single camshaft operates 24 underslung rocker arms on top of each cylinder bank -- twelve rockers pivoting from a rocker shaft on the intake side of the block to actuate the exhaust valves, the other twelve rockers pivoting from a rocker shaft on the exhaust side of the block to actuate the intake valves. The camshaft is carried in seven forged aluminum alloy brackets, each bracket fitted with a bronze bushing at the rear to support the camshaft. The rocker arms have contact pads which are faced with hard chromium, and have screw-type adjusting tappets which directly contact the valve tips. The camshaft is driven by an inclined drive shaft which connects to the upper bevel gear of the wheel case. The inclined shaft drives double spur pinions which turn the camshaft at 1/2 engine speed and provide drives for various accessories which can be mounted to the supercharger ends of the cylinder heads. Pressure lubrication to cam bearings is supplied through the hollow camshaft.

Wheel Case: The wheel case assembly is mounted to the supercharger end of the crankcase and contains gearing and shafts that drive the camshafts, magnetos, supercharger, starter, generator, and oil, fuel, and coolant pumps. The magnetos, starter, coolant pump, and fuel pump are mounted to the wheel case. The crankshaft is connected to the wheelcase via a spring drive which consists of a torsionally resilient inner shaft which connects to the crankshaft via a splined coupling and a stiff hollow outer shaft which is coupled by splines to the other end of the inner shaft. The wheel case outer drive shaft carries the supercharger driving gear and the upper and lower vertical drive bevel driving gear. A sleeve splined onto the outer shaft also fits into the splined crankshaft coupling with an allowance for six degrees of relative motion between the crankshaft and the hollow shaft, limiting the maximum relative motion between the two.

The upper vertical drive carries three gears -- the lower bevel gear meshes with the bevel driving gear on the wheel case outer drive shaft, the center helical gear operates the magneto drive, and the upper bevel gear drives the camshaft drive train via two inclined drive shafts (one for each bank). The lower vertical shaft also carries three gears -- the upper bevel gear meshes with the bevel driving gear on the wheelcase outer drive shaft, the center gear is a helical gear which engages the fuel pump driving gear, and the lower spur gear which drives the oil pump idler and hydraulic pump gear train. The shaft is also internally splined to accept a quill shaft which turns a cooling pump coupler.

The generator drive, aftercooler pump, and starter motor drives connect to the crankshaft via pinions for the supercharger planetary gears which engage with the supercharger driving gear. The generator drive and aftercooler pump are driven by a spur gear engaged to the "B" side supercharger planetary pinion. The starter drives the crankshaft through a reduction gear train coupled to by a spur gear engaged on the "A" side supercharger planetary pinion.

Supercharger: The supercharger is a two-stage two speed gear driven unit with intercooling and aftercooling. Two stages are used to obtain relatively high pressure ratio's efficiently, two-speed operation allows improved performance at high altitude without the loss of power at sea-level which is inherent at high blower speeds, and intercooling and aftercooling significantly reduce charge temperature, allowing higher boost without detonation.

For low speed operation, the supercharger turns at 6.391 times crankshaft speed, for high speed the blower spins at 8.095 times crankshaft speed. The supercharger is driven through a gear train coupled to the spring drive through the supercharger driving gear in the wheelcase. Three independent planetary gear trains are arranged at 120 degree intervals around the driving gear, the driving gear thus driving three planetary pinions. The ring gears for the three planetary drives have internal and external teeth, the external teeth are coupled to the supercharger pinion which directly drives the supercharger, the internal teeth are coupled to planetary gears coupled to planetary pinion gears. For low speed, the planetary pinions are directly locked to their respective ring gears through planetary gears and clutches, so the ring gear is turning at the speed on the input to the planetary geartrain. For high speed, the planetary gears are allowed to rotate about the sun gear, increasing the speed of the ring gear with respect to the input shaft. A hydraulic clutch (three really, one for each planetary gear train) combined with over-running "sprag" clutches is used to effect speed changes. The hydraulic clutch uses moderate engine oil pressure controlled with 24V electrically actuated solenoid -- the solenoid is actuated to apply oil pressure for high speed, and pressure is released for low speed operation.

The supercharger itself consists of two impellers on the same shaft, both turning the same speed. The first stage uses a 12.0" diameter impeller, while the second stage uses a 10.1" impeller. The intercooler is an integral part of the intermediate volute case, located between the first and second stage impellers. The compressed air from the first stage passes through the cooled volute and passage to the second stage. The aftercooler is located between the exit of the second supercharger and the intake plenum, and is a conventional air/water heat exchanger (liquid/air radiator). A separate cooling system with its own pump was provided (permitting cooler water than is possible by using the engine coolant) was provided for aircraft use -- a 40% reduction in intake temperature was reported by Rolls-Royce at maximum speed and power with a coolant flow of about 30 gallons per minute. For boat racing, the aftercooler is usually replaced with simple plenum tube (called a tube or ADI tube) due to disruptions in airflow and mixture which occur in the aftercooler matrix at very high power levels -- at least some of which is though to result from air-fuel-ADI separation.

An automatic boost regulator is standard equipment for aviation use. The boost regulator automatically retards the throttle as full boost is reached, eliminating manual control of this critical function. War emergency boost is often provided whereby the pilot can over-ride the regulator when necessary, an indication such as a broken seal is usually provided to alert maintenance that the engine has been overstressed. For planes with ADI, there is usually an interlock provided that restricts maximum boost to a lower setting when ADI is not functional.

Two stage supercharging permits higher blower pressure ratios to be developed efficiently, which improves high altitude performance. Similarly, the high blower speed was used as atmospheric pressure fell to increase altitude performance. At sea-level, high blower ratios could not be used because too much power was consumed by the supercharger. At high altitude with low density air, the speed could be increased since the amount of power used by the supercharger decreased dramatically as the air density decreased. Because of this, low blower is almost always used for racing applications. (The two speed blower reason to exist is to provide compensation for the dramatic difference in air density at altitude, for a limited range of air density like sea level condition the blower ratio is fixed and there is no reason to have two speeds).

Induction: A Bendix Stromberg PD18 injection carburetor is used on the V-1650-9 engine. This double-barrel twin-boost venturi carburetor has an air metering unit consisting of the two throat throttle body with boosted venturies, a fuel metering unit consisting of a regulator which provides a fuel head proportional to air flow and metering jets which provide fuel flow proportional to fuel head (and thus airflow), and an injection nozzle which directs the discharge of fuel into the eye of the supercharger and provides a reference metering pressure for the fuel metering unit. The system includes and accelerator pump to provide fuel during momentary interruptions as power settings are changed, an automatic mixture control which adjusts the airflow metering signal to compensate for temperature and altitude, a manual mixture control which changes jet selections and bypasses the automatic mixture control, an automatic fuel enrichment valve to increase fuel flow under high demand, and an ADI derichment valve which leans the mixture when a pressure signal is applied to the valve.

Fuel pressure is from an engine driven eccentric-vane type pump with an integral pressure regulator. An electric boost pump of the same type provides initial priming and for racing applications runs while the engine is on.

Engines using ADI (anti-detonation injection, usually 50:50 mixture of methanol and water) use some form of an ADI regulator to generate an appropriate flow of ADI for the engines operating condition. While various ways of doing this exist, most boat racers use a system developed by Dixon Smith Systems, Inc which provides an ADI regulator which is controlled by boost pressure. A needle valve inside the ADI regulator opens as pressure increases, the needle valve is shaped to provide the required flow for a given boost setting. Eccentric-vane type pumps provide relatively constant ADI pressure to the regulator regardless of demand, and the derich valve of the carburetor is activated when the ADI system activates. Each type of engine requires an ADI regulator calibrated for it, since the specific ADI consumption at a given level of boost is a function of engine parameters. Generally, ADI systems are calibrated to provide about 1/2 pound of ADI per pound of fuel consumed. ADI is used because it provides very good charge cooling, dramatically reducing detonation at high boost levels. ADI is usually injected into the eye of the supercharger (often its just a pipe extending into the intake elbow). ADI is very corrosive and is very hard on pumps, hose ends, sensors, etc ... frequent flushing and pickling with water soluble oil is required.

Air-fuel mixture passes through the two stages of the supercharger and through the ADI tube (or aftercooler) to the intake plenum which sits deep in the "vee" of the engine between the two cylinder banks. From here, the mixture is distributed to each bank by an intake plenum which bolt directly to the intake ports. Flame traps are contained within the intake manifold immediately adjacent to the intake ports which contain backfires and flames which could devastate the plenum if its high pressure mixture were to ignite. The intake manifolds also provide atomizing priming jets which are connected to a solenoid on the PD18 metering unit to facilitate starting by direct injection of fuel into the intakes.

Ignition: Ignition is by two high-tension rotating magnet magnetos mounted on each side of the wheelcase. The "A" bank magneto serves the intake spark plugs and the "B" side magneto serves the exhaust spark plugs. The magneto has an integral coil and breaker points operating off a four pole cam, each magneto is driven by a serrated coupling shaft turning at 1.5x engine speed. A distributor turning at 1/3 the magneto speed provides one complete cycle of 12 sparks every 2 revolutions of the engine. Provisions exist for an engine control mechanism to move the breaker base through 25 degrees of advance. In addition, the distributor features a two contacts on its rotor -- the leading finger is energized by the magneto and the trailing finger by an external terminal which can be connected to a booster coil to facilitate starting. The booster coils are essentially high tension coils with vibrating points which provide continuous spark -- when applied to the retarded finger they provide a high energy retarded spark which can greatly facilitate starting, especially with cold, slow turning (thick oil) engines which don't provide much speed for the magnetos. Merlins in aircraft only boosted the "A" bank, but I've seen boats with no boost, with booster coils on both banks, and both combinations in between.

Lubrication: Lubrication is by dry-sump pressure system incorporating three pressure stages -- main, moderate, and low. Circulation maintained by a single pressure pump and two main and one auxiliary scavenge pumps, all of the gear-pump type. Pressure is regulated by a pressure-sensitive balanced relief valve mounted on the "A" side of the crankcase and oil is delivered to the relief valve unit via an external Cuno-type oil filter. The main pressure is 60-90 psi and is supplied to the crankshaft to lubricate the main and connecting rod bearings, and to the propeller constant speed governor. The moderate pressure section is distributed at 28-32 psi to lubricate the supercharger impeller and clutches bearings and to provide operating pressure via the supercharger speed solenoid of those pressure clutches. The low pressure stage lubricates the reduction gear, valve train, and generator drive. Splash lubrication is provided to the cylinder liner walls, piston pins, oil pump idler bushings, reduction gear bearings, wheelcase including upper and lower vertical drive shaft gears, lower inclined drives, gear change mechanism, and starter.

The scavenge oil system uses two pumps -- one to collect oil at the propeller end of the crankcase, and the other to collect oil at the supercharger end of the engine. An small auxiliary scavenge pump is located directly underneath the main pressure pump and scavenges oil from the supercharger rear bearing into the lower crankcase sump through the hollow driving shaft. The scavenge lines generally contain additional Cuno filters between the pump and the oil-cooler or dry-sump tank.

Coolant: The typical cooling system employed a mixture of 70% water and 30% ethylene glycol. The coolant in the closed loop pressurized system is circulated by a centrifugal-type pump to the cylinder blocks and from the cylinder blocks to a small-capacity header tank and from the header tank via a radiator to the coolant-pump inlet. The flow of coolant air through the radiator is controlled, whether manually or automatically, through a temperature-sensitive device which controls radiator shutters. The header tank, which incorporates features to ensure the efficient separation of steam and coolant, is provided with a loaded relief valve which seals the whole coolant system up to a predetermined pressure. This pressurizing of the system raises the boiling point of the coolant and permits the use of smaller radiators. The header tank relief valve maintains the pressure in the system and also incorporates a suction-operated valve which admits air, if for any reason the pressure falls below atmospheric.

For boat racing, a total loss system is used with pickups aft of the sponsons (old way) or at the bottom of the rudder (current method). The water is usually routed to a water-oil cooler, then distributed to the two banks. The water pump is generally not used (but could be handy if slow operation were desired). The banks then drain overboard. To avoid steam, it is useful to build a little pressure, so the outlets are sized appropriately. Water is also used for the shaft logs and for intercooling/aftercooling. A separate pickup for use with a water speed indicator is also common.

Starting: Electric starter motor composed of 24V series wound electric starting motor engaging reduction gear at bottom of wheelcase (motor mounted vertically). The geartrain provides a reduction more than 104:1, and includes a safety clutch and a modified Bendix-type mechanism to protect the drivetrain and related components from excessive torque and to allow the starter to disengage while the engine is running. Booster coils are employed to retard spark and increase ignition during low-speed cranking. Primer system for direct injection of fuel through atomizing nozzles into intake manifolds provided.

Propeller Drive: The reduction gear unit is bolted to the upper crankcase at the end opposite the supercharger. The reduction gear provides a ratio of 0.491:1, using a 21 tooth spur-type drive pinion, and a 44 tooth propeller shaft gear. The drive pinion is supported on both sides by straight-type roller bearings and driven by the crankshaft through a hollow coupling shaft which couples to internal splines in the crankshaft and internal splines in the pinion. Likewise, the propeller shaft is supported on both sides of the propeller shaft gear with straight roller bearings, with an additional ball bearing on the propeller shaft to take the thrust loads.

Engine Models and Applications: For a complete listing of the Merlin/V1650 models and derivatives, readers are suggested to see Daniel Whitney's "Vee for Victory!" or Graham White's "Allied Aircraft Piston Engines of World War II".