It is not generally known that the first airless injection system (i.e. not to use compressed air to atomise the fuel) was a common rail system. The invention of this system is often mistakenly credited to Doxford, but it was invented and patented by Vickers of Barrow in Furness.
In this early common rail system the engine driven fuel pumps pressurised a fuel rail to about 400 bar from which pipes led to the fuel valves operated by cams and rocking levers. Independently driven pumps were provided to prime the system for starting.
Later systems used hydraulically operated injectors, the delivery of fuel being controlled by a cam operated valve. Fuel quantity was controlled by an eccentric on the cam follower.
With the integration of industrial electronics into marine engineering systems coupled with the giant strides made in the development of computer technology, it has now become possible to re-introduce the fuel injection common rail along with other fuel injection systems, using this modern technology to time the injection of fuel without mechanical aids.
In addition to this, it has become possible to dispense with the timed camshaft altogether by using similar systems to control operation of valves and the air start system.
The two major manufacturers of two stroke crosshead engines have both introduced a camshaft-less engine. Sulzer call theirs the RT Flex engine, and MAN B&W call theirs the ME intelligent engine. Both engines use electrical and engine driven axial piston pumps to pressurise servo oil rails to 200 bar which are then used for fuel injection and exhaust valve operation. In addition MAN B&W use the servo oil to drive the cylinder lubricator units (Alpha system)
Although they both work without a camshaft and use computers to control, fuel injection, exhaust valve operation and air starting, the method of fuel injection is different.
Sulzer use a pressurised fuel rail using a set of jerk type pumps driven by a three lobe cam geared to the crankshaft. The pumps are variable delivery, based on the ZA40 fuel pump, controlled by an electrically driven fuel pump shaft linked to the engine computer.
The engine computer system known as the Wartsila Engine Control System (WECS)controls the delivery from the common rail to the individual cylinders via the volumetric injection control system which uses finely filtered engine LO pressurised by electric pumps to 200 bar.
When the Rail Valves are energized for injection by the Valve Driver Module, oil from the Control Rail opens the Injection Control Valves. The fuel injectors are pressurized and fuel oil pressure behind the Fuel Quantity Piston maintains this pressure at the injectors. As the Piston moves to the left a feedback signal is sent to the Cylinder Control Module.
At low engine load the control system cuts out one of the three injection valves per cylinder.
At very low load two of the three injection valves are cut out. This is used to avoid visible smoke emission and to reduce fuel consumption. It is possible to reduce engine load to 10% with engine revolutions as low as 7RPM.
Unlike the Sulzer RT Flex engine the MAN B&W ME engine does not operate the fuel injection on a common rail system.
Instead a solenoid operated proportioning valve (the FIVA valve - Fuel Injection Valve Activation) allows the pressurised servo oil under a hydraulic piston. This then moves the fuel pump piston upwards, raising the fuel pressure and opening the injection valves.
A nitrogen filled accumulator maintains the hydraulic servo oil pressure during the operation of the pump.
To be able to time the fuel injection the Control Systems must know the crank angle of the individual units. To do this two crank angle sensors are fitted at the free end of the engine. These sensors are accurate to 0.1°. Cylinder pressures and powers are continually monitored by using strain gauges built into the cylinder head, and the computer automatically compensates for twist in the crankshaft when relating crankshaft position to cylinder pressure. the systems give complete flexibility over start and end of injection and take into account fuel quality, dead time (the time between injection start command being given and actual injection), and Variable Injection Timing (VIT)
The exhaust valve actuator replaces the cam operated exhaust valve hydraulic pump on both make of camshaftless engines. Both working on a similar principle, servo oil at 200 bar is used to operate a piston which operates the exhaust valve "hydraulic push rod" The oil for operating the "hydraulic push rod" comes from the main engine LO supply via a non return valve.
The air start system is similar to that on a conventional engine except there is no need for a mechanically driven distributor to open the air start valves at the correct time.
Instead of a camshaft driven, reversing air start distributor, each air start valve is opened at the correct time by the engine computers sending a signal to a solenoid controlled nc (normally closed) valve.
The timing of the air start valves will vary depending on the number of cylinders, but they will be open for a long enough period to allow overlap, so that a valve opens before the previous valve closes, allowing starting from any position of rest. The nominal opening can be considered as 0° (ie TDC) and closing at 110° ATDC.
The computer knows when to send the signal because it is receiving information as to the crankshaft position from the angle encoders which measure crankshaft position and RPM.
When the engine has reached firing speed the computers shut off the air and introduce the fuel.
This gives a brief overview of the computer controlled camshaftless engine. More detailed explanations with detailed drawings can be found in the members section under Common Rail and Camshaftless Engines
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