Archive for the ‘Island mode’ Category

Generators – Gas and diesel engines can not be used the same way!

Sunday, April 11th, 2010

The simple reason diesel engines respond much faster to load transients than gas engines is because fuel introduction on a diesel engine is done directly into the combustion chamber, at the next engine cycle from a load change, if the control system is fast enough, the different fuel rate can be introduced.  On the gas engine, fuel is introduced upstream, on older engines at the carburator mixer, on newer engine its at the valve, on medium speed gas engines at the inlet port of the cylinder head.  All points of fuel introduction produce delay before the cylinder sees the desired fuel change.

Gas engines do have a number of issues operating “island mode” when compared to diesels.  Properly designed, applied and integrated systems can operate effectively as prime applications, including maintaining very low emissions with installed after-treatment and heat recovery systems.

Transient capability is VERY different between gas and diesel.  Newer design gas engines not only have issues with load acceptance, but in many ratings, have a worse time with load rejection.  Also, gas engines have significant degradation in transient capabilites between “normal” service intervals.  Issues such as valve lash, spark plug condition, and ignition component health, like extenders and transformers, all affect engines ability to respond to load changes and maintain stable operation.  Also, newer engines have very different response characteristics depending on load level, low load pickup on some engines is extremely poor, mid range load pickup can be best if sufficient turbo response is available.  Top end transient response can be erratic as control system limitations, emission control and available turbo response all can fight each other.  So a 25% load transient may be acceptable at 25-50% and 50-75%, but unacceptable at 0-25% and 75-100%.  It is also quite possible the engine will not tolerate a 25% load rejection.

Also be aware that the applied protection settings for voltage and frequency deviation may not allow for desired transient operation, and using volts/Hz for improved recovery may affect operation of system loads like VFD’s and UPS systems.

New gas engines also have a much harder time with running extended periods at “low load”, actual load levels that would be defined as low load can vary, but gas engines suffer increased problems with spark plug life, especially multi-torch type spark plugs due to accelerated deposit levels.  Increased cylinder deposits also affect engine combustion, detonation levels, and emissions outputs, so assuring the engines are properly matched to the system load profiles is essential in maintaining stable plant operations.

As pointed out above, older engine designs with simpler and more robust controls systems were easier to apply in island applications, newer engines needing to meet reduced fuel consumption, reduced emissions and higher power densities have to give up something, and transient response suffers in these engines.

Island mode covers a fairly broad topic, engines can be successfully operated in prime, peaking and non-critical applications. From past experience, I am firmly opposed to using a gas engine as a critical standby unit for life safety.  Current design gas engines have a huge number of shutdowns programmed into their ECM’s, a large number of these designed to protect the engine, adding complexity and reducing reliablity.  Gas engines have ignition systems, spark plugs on cylinders open to the atmoshpere corrode, ignition wiring deteriorates, tranformers get internal faults, and ignition sources, such as magnetos or ignition modules can appear to operate correctly at no or low loads during testing and fail when called to operate at higher loads.  Fuel systems components, such as gas regulators and carburators, can stick and bind, diaphragm materials deteriorate, springs fatigue.  Even newer fuel systems components, such as the Raptor valve have relatively high failure rates.  While no engine is 100% reliable.  Gas engineers are known to have a very large number of fails to start or failure to operate as expected with gas engines in standby service as compared to a much larger population of diesel engines.

Compounding the problems with using a gas engine as a critical standby unit are two issues, in my opinion.  First, planning restraints regarding air quality control in conservation areas and noise in built up area shave greatly reduced the number of available hours a unit can be run for maintenance.  Second, most customers don’t want to run their unit under load, while some do install permanent load banks or do regular site load testing, their number is small compared to the total population.  And since most standby systems are low cost installations, good monitoring and trending systems for engine mechanical or electrical parameters are likely not installed.  So these engines don’t run enough hours at a high enough load in a year to assure their engine systems are functioning correctly.  So if you’re going to apply a gas engine in critical standby service, you have to be aware it has a higher incidence of failure, needs more maintenance, and can have reduced performance in between service intervals than a comparable rated diesel engine