Engine misfire largely to blame for vibration onboard

Sep 2, 2016 by Rich Merhige

Vibration is easily attributed to misalignment, a bent shaft or a deficiency with a propeller. Often, though, none of these assumptions are true, and the true source of the problem is engine misfire.

The combustion process is one of the most important functions of a diesel engine. It is comprised of several physical and chemical reactions, and it has four stages:

  • 1. Ignition Delay. This is the period from the beginning of the injection to the point of the ignition of the fuel. The fuel that’s injected must be completed vaporized, the air-to-fuel ratio must be precise, and the temperature must be high for ignition to commence.
  • 2. Pre-Mixed Burning Phase. As ignition begins, heat spreads rapidly throughout the engine, causing combustion to begin.
  • 3. Diffusion-Controlled Burning Phase. Lower heat is diffused to control the burning of the fuel.
  • 4. Combustion Tail Phase. The falling temperature and pressure initiates a cooling process, resulting in slower heat release.

An engine misfire occurs when one or more of the cylinders inside the engine doesn’t fire correctly, especially when there is an interruption of the air-to-fuel ratio inside the combustion chamber in the cylinder.

Engine misfire causes the power to drop and reduces vessel performance. It also increases emissions and can cause severe vibration throughout the vessel. Misfires can happen constantly or intermittently.

There are actually three categories of misfires: fuel, ignition and engine mechanical. In diesel engines, a misfire is caused when the ignition is bad. Bad ignition can be premature, delayed or incomplete combustion.

It’s important to note that misfire doesn’t always occur at all RPMs. It could actually occur at idle.

Other scenarios that can make a misfire more likely to occur include:

  1. A lack of air getting to the engine make it difficult for the chemical reactions necessary for combustion to happen.
  2. Faulty electronics for a fuel system can hamper the flow to the engine, hampering the reactions necessary for combustion.
  3. Fouled injectors can lead to poor atomization.
  4. Poor timing.
  5. Contaminated or poor quality fuel.

Misfires are one of the most difficult problems to diagnose and correct because there are so many situations that can cause it. When an engine misfires, performance suffers. When performance of a mechanical component suffers, so does fuel economy and power output. This also increases emissions by reducing the efficiency of combustion.

The cylinder cutout test is often used to diagnose a weak or failed injector or a misfire that could be caused by something other than the injector. The test disables each injector and measures the difference in the delivered fuel volume with the injector disabled compared to enabled.

With the cylinder cutout, load on the remaining cylinders is higher and the delivered fuel volume increases to compensate for the disabled cylinder. If a failed injector is cut out, delivered fuel volume on the remaining cylinders will not change. You will get the same results if a cylinder is not firing due to some other mechanical problem, such as leaking valves or piston rings. Performing the cylinder cutout test with a load on the engine tends to give more accurate results.

Another tool for diagnosing misfire is vibration analysis. When a general hull and machinery vibration survey is conducted, the engine misfire vibration is picked up as a half-order harmonic of the engine RPM.

This vibration is usually absorbed through the engine’s isolators, but excessive misfire may be transferred through the isolators or exhaust system and into the hull.

A traditional vibration analyzer will not pick up the exact cylinder or cylinders misfiring nor identify the exact nature of the misfire. To more precisely diagnose the misfire, use a diesel engine analyzer that uses advanced crankshaft referenced vibration and ultrasonic measurements across different vibration ranges on each cylinder to identify and detect change and degradation to internal engine components.

By identifying specific issues, they can be fixed before they become unexpected, unplanned and expensive repairs. If left untreated, engine misfire can cause imbalance or overloads to the crankshaft journals and connecting rods, which can be detrimental to an engine’s life. This would mean having to conduct complete engine overhauls much earlier than usual and replacing major components.

Today, diesel engines are technically advanced and built to survive in demanding environments. However, these are large machines that have thousands of parts and multiple systems that operate at high speeds. When any one component degrades or fails, it can lead to a host of negative or catastrophic events.

Most engineers follow the prescribed maintenance and routine inspections, but even with that, a significant amount of maintenance expense goes into unexpected failures. Condition monitoring programs with tools such as vibration analysis can help identify problems before they cause failures.


Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale (www.AMEsolutions.com). Comments are welcome at editorial@the-triton.com.