What happens when you apply load to a fixed speed diesel engine, such as used in a generator application?

When referring to the application of load to a fixed speed diesel engine, we are actually talking about additional torsional resistance being applied to a running engine and how that engine then reacts to overcome that force.

When an engine is running at a set speed it will produce a set torque figure. When the load is applied the engine will effectively react in one of three ways, depending on the size of the load:

1. If the load is small the force of the engine should overcome the force of the load easily and with little noticeable change in RPM or change in air fuel mixture.
2. React to the load by applying extra fuel and air mixture, which will create extra power and overcome the load. This can often manifest visually in a diesel engine by a short drop in RPM and initially an inability to burn the extra fuel cleanly which manifests as black smoke.
3. The engine may stall if the load is too great for it to react to or simply too great for it to turn.

When a load is applied to an engine we refer to it reacting to that load. To give this term more detail an engine reacts by increasing the amount of air and fuel that is taken into the cylinder on each operating cycle. The greater the air fuel mix, the bigger the explosion, the more power is produced and subsequently increased to the point where the engine can overcome the load.

In modern diesel engines the reaction to load is handled electronically and through ECU managed fuel injection. In older machines mechanical fuel injectors were used in conjunction with a mechanical governor that reacted to a ‘drop’ in engine revs caused by the load application.

Naturally aspirated engines, due to their configuration are inherently better at reacting to load in comparison to turbo charged engines. This is because with turbo engines there is an extra phase in the reaction which is a delay caused by the need of the exhaust gases from the engine to spool the turbo before air fuel mix can be effectively increased.

As such with modern turbocharged Diesel engines in particular (although in reality for all engines) it is imperative that load is applied in stages and in manageable amounts. An engine may well be able to overcome a load but may not necessarily be able to react to that load quickly enough or in one large step.

For increases in load that are greater than trivial in size you can hear the engine sound change as the speed initially falls and then recovers, assuming the engine is not now overloaded. At level of load greater than zero, the engine will be louder, with the sound level peaking at its maximum load. This additional sound energy comes from the larger more powerful explosions of diesel and air in the engines cylinders,  needed to provide the additional energy to rotate its load. If the load is provided by an AC alternator then the alternators magnetic field, between the main rotor and main stator, becomes stronger and its resistance to turning increases.

This is often referred to in the power generation industry as successful load acceptance. This is key to not only producing the required results but also minimizing high resistance torque variations which can significantly wear and damage the engine.

As such when considering the size of engine required for a task and the amount of load that will be required; the step loading of that engine must be considered alongside its maximum power outputs. In addition, it is critical to consider a starting procedure for loads applied to ensure the engine is not overburdened or damaged.

If connected to an AC alternator making a diesel generator, then the load will also effect the AC alternator.