Unit injector vs Common rail in the marine vessel industry

Depending on what area you operate in, there are different needs and requirements on the equipment. To achieve maximal output on marine vessels, it is important to use diesel engines specifically adapted to the operating conditions. In this blog article, I will describe the two fuel injection systems Unit injector and Common rail, and the differences between the two.

Requirements on diesel engines in the marine vessel industry
here are a couple of main reasons why professionals working with marine vessels have tough requirements on their diesel engines. As climate change and pollution are becoming an increasing threat to our planet, it is more important than ever to comply with current emission legislation. A smart and fuel-efficient engine is therefore a must. Another aspect that the marine vessel industry is dependent on is the engine’s reliability. Unplanned downtime can be devastating for a business’s operations. To feel secure and safe during the engine’s whole life span is therefore crucial. Many diesel engines working in the marine vessel industry often perform at high speeds and power outputs, long periods at a time. For the engines to deliver excellent results and work at full capacity, they need to be robust, durable and maintain a high quality.

Fuel and air – two crucial factors a diesel engine need to function
Fuel and air are the two most basic and necessary things that a diesel engine needs to function, since oxygen molecules are required for the fuel’s combustion process. The combustion takes place when the piston compresses the air in the combustion chamber and the temperature rises to ignition temperature. When the fuel is injected, it ignites, causing the engine to run. Central to the diesel engine is the fuel injection system. It regulates the engine’s power and has a major impact on the efficiency and exhaust emissions.

A high pressure in the fuel injection system is beneficial for the environment
The unit used to measure the pressure in the fuel injection system is the metric unit bar. The higher the pressure in the fuel injection system, the more friendly it is to the environment. High pressure results in small fuel droplets with high energy which are needed for fuel and air to mix for clean combustion. For high power engines, a high quantity of fuel needs to be injected during a short period. To achieve this, a specific nozzle flow is selected. Normally, the main injection has to end at crank angles between 24 and 27 degrees, before the piston expose the cylinder wall. The surface of the inner wall is cold due to cooling water outside of the cylinder. That means the small amount of fuel that hits the wall will not be combusted and the lubricating oil will be diluted. If the lubricant disappears, the engine may soon be worn out. In addition, unburned fuel and soot will enter the engine oil leading to oil degradation. Long-term this will be costly for the vessel owners with the risk of engine failure and high service costs.

This is one of the reasons why one should never try to tamper with the power setting of the engine. To prevent this from happening in the future, all injections outside the area where the cylinder walls are exposed will be cut into a series of very small injections to avoid the individual injections to hit the cylinder wall.

Unit injector vs Common rail
There are several fuel injection systems, where two of the most common systems at Volvo Penta are Unit injector and Common rail.

Unit injector – the injector and pump in one single component

The Unit injector is an integrated direct fuel injection system, combining the injector nozzle and the injection pump in a single component. A camshaft usually drives the plunger pump. Instead of having a constant pressure on stand by as with the Common rail, the pressure is recreated in the injector before every injection. The Unit injector system has a peak pressure of up to 2 500 bar. Due to short lines and few constrictions between the pump and the injector, the Unit injector create an effective injection process.

A smooth system that decreases emissions
The Unit injector has a triangle-shaped injection rate, which enables a small amount of fuel to be injected before the combustion starts, which contributes to lower emissions. Early in the process, the Unit injector injects a smaller amount of fuel, compared to the Common rail. When the combustion process has started though, it is important to maintain a high fuel supply. The unit system’s’ strength is to have a low supply of fuel in the beginning and then increase the supply at the exact right moment when the combustion starts. This enables a high efficiency of the engine, keeping the soot and emission levels down.

The downside with Unit injectors is the lack of flexibility as well as the fuel quantity because of the limitations set by the camshaft. Common rail is much more flexible in these aspect since Common rail does not rely on the camshaft for each injection.

Common rail – a common fuel rail feeding individual injectors
Common rail is the latest fuel injection technology, featuring a high-pressure fuel rail feeding individual injectors. The rail is filled with fuel at a very high pressure, consistent with the pressure that is needed for each injector. The injectors inject the fuel through small holes in the nozzle into the inside of the cylinder. The rail is connected to a common pump and as soon as one of the injectors has injected fuel, a continuous refill from the pump takes place. The refill, regulating the rail pressure and the pump’s workload, differ depending on what pressure you need and when. Across different work areas you will constantly need different pressures. The pressure of a Common rail system is between 1 600 and 3 000 bar.


A system that uses the engine’s full capacity
A benefit with the Common rail system is the high pressure that uses the engine’s full capacity. Because the fuel pressure is stored remotely and the injectors are electrically actuated, the injection pressure at the start and end of the injection is very near the pressure in the rail, thus producing a square injection rate. The initial fuel amount injected before combustion start is larger compared to Unit injector.

To overcome this, two methods is being used. The first method is a restricted orifice used in order to reduce the initial fuel rate that is installed between the rail and injector. The restriction will also reduce the injected fuel to the engine during the available time. The restrictions are carefully selected to dampen pressure waves that can cause irregular injection behavior. The second method is several small injections. By introducing extremely quick and responsive injectors, modern common rail systems reduce the initial amount of fuel rate by a number of small injections close to each other. At the point of ignition, the main fuel amount is then injected into the cylinder at high pressure and minimal restriction. This can be done with full flexibility in timing, enable the common rail system to overcome the limitations with Unit injectors. The internal components have to be selected and tuned carefully to avoid pressure waves causing irregular injections.

Volvo Penta delivers customized high-power solutions that focus on the environment
Volvo Penta uses both Common rail and Unit injector in its engines. The Unit injector is an excellent injection system for many engines. It is effective, consists of fewer components and connections and is cost-effective. A Common rail system is suitable in operations where flexibility is required from the injection system. Engines that are in need of a Common rail system are often engines where a high power output is required while keeping the emissions low, for example in gensets. Although both Common rail and Unit injector generate low emission levels, the Common rail system combine high pressure with total flexibility of timing and is therefore the system of the future. In combination with the use of EATS (Engine After Treatment Systems) to further reduce emissions, a flexible fuel injection system is needed. That is why the Common rail system will be used more and more frequently in Volvo Penta’s engines.

Volvo Penta focuses on delivering customized high-power solutions that are specifically designed for your core business and needs. The environmental aspect is always high on our agenda, as we manufacture our products to generate as little environmental impact as possible.