Generator Set Engine Selection Tips

Here I would like to share some engine selection tips for you to use when purchasing a gen-set.

Large, reputable dealers will run through these as questions during the sales process.  In fact this is a bare minimum for questions when I am selling a generator.

But some smaller suppliers may just sell you what they have on the shelf, without considering the consequences over the lifetime of the unit.

A little research and calculation at this stage will ensure that you are satisfied with your equipment purchase.

Application Type

Here are some engine selection tips to use, depending on the type of generator application you have.

Are you purchasing for use as an emergency stand-by generator for a commercial building?

A unit like this is likely to run for less than 50 hours or 100 hours per year, including all exercising and testing.  I would suggest that you could live with a higher fuel consumption and higher engine speed.

Important will be reliability to start, lower capital costs, and higher power density.  Price and footprint size are the name of the game here.

Will the generator be used for prime power applications – say to power a construction site, work camp, or to parallel the grid for peak shaving, grid feeding, CHP co-generation?

Now there are more things to consider.  You should seek to minimize fuel consumption, engine speed, and power density.

These units run for over 5000 hours per year.  Over the life of an engine, the cost of fuel greatly outweighs the initial capital expenditure for the equipment.

Durability and reliability are extremely important.  Having a unit down for service or repairs can be costly, especially when you require a rental unit to take it’s place.

Here is a refresher on engine power rating definitions.

Fuel Type

In the past, almost all generators were diesel powered.  Diesel is the most dense and easily transported form of chemical energy that you can use for a generator set.

However, it has become very expensive, and has made other fuel sources very attractive.

Will the unit be near to a natural gas pipeline?  If so, then seriously consider this as the fuel for a prime power generator.

Q: Why only for  prime power?

A: Natural gas engines tend to be less power dense and more expensive than their diesel powered brothers.  On a prime power unit, the increase in expense is outweighed by the fuel savings.  Do your ROI calc!

Also, natural gas isn’t reliable enough for emergency back-up power.  During a natural disaster, the gas lines tend to be the first utility that is interrupted.  Life critical stand-by power must remain diesel.

Finally, serious consideration should be made to utilize any of the following fuel sources that are available to you:

• Landfill gas
• Wellhead gas (i.e. at an oil well)
• Flare stack gas
• Livestock waste methane

These fuels are essentially free and can be ideal for combined heat and power co-generation.

Fuel Consumption

A major engine selection tip of mine is to look at the lifetime cost of fuel

In a prime power generator application, the lifetime cost of fuel is many times greater than the initial capital expenditure for the equipment.

I’ve been involved in enough power house applications to know this.  Sometimes you even have to provide warranted fuel consumption specifications to the customer.

If the package you built and delivered is thirstier than you quoted, you can be on the hook for the difference in fuel costs.

As a generator purchaser, be sure to do your own calculations if fuel consumption is important to you.

Take a close look at all predictions made by engine and gen-set manufacturers.

When making comparisons, make sure that variables used are the same for all engines – or at least understand what made the differences.

Fuel density, fuel heating value, parasitic losses, generator efficiency, and fan power draw are parameters you should look at.

I have seen engine specification sheets with abnormally high fuel densities.  This helps the volumetric fuel consumption appear artificially low.

Engine Speed

Engine speed plays a major factor in:

• Fuel Consumption
• Noise
• Durability and longevity
• Power density and footprint
• Maintenance intervals
• Capital cost

A 900rpm or 1200rpm continuous power generator will have the lowest fuel consumption and will essentially run forever.

However, the power density is very low, requiring a very large footprint.  Also, the upfront costs of this large equipment will be noticeably higher.

The industry standard for 60Hz power between 15 kW and 1500 Kw is 1800rpm.  These units create more noise, burn more fuel, and require more frequent maintenance.

Some smaller backup power systems run at 3600 rpm.  Fuel consumption is high and life is short.  However, they are very power dense and are cheaper to purchase.

Cooling System

Here is an often overlooked engine selection tip for you.  Reviewing the cooling system requirements is less commonly used as a engine selection criteria than the others listed here.

To be honest, this is less of an issue with smaller units that have an engine mounted cooling package.

However, the type of cooling can be a big deal if you are looking to remotely mount the cooling components; whether on the other side of a wall, or on the roof of your building.

There are some exceptions, but most engines that you will consider will be liquid cooled.  The exceptions being some <100kW engines that are available air cooled.

Though that are initially simpler, air cooled engines require massive amounts of air movement, are very stressful on the lubricating oil, and can be difficult to package in an enclosure.

Mounting a cooling package on the roof of a building can be a great way to get the required airflow and reduce the effects of fan noise.

In this situation, look for an engine that does not have a separate charge-air-ccoling system – a difficult task in this age of emission regulations.

Even single circuit systems require a diligent eye:

• How high will the cooling package be?
• What is the pressure head on your engine?
• Will the radiator hoses be ok with this pressure?
• Can the engine’s water pump push the coolant that high?
• Has the pressure cap on the radiator been changed to reflect this pressure head?
• Is a separate circuit required to get cooling water to the roof?
• Now size a heat exchanger and water pump
• Is there a large enough expansion tank to take up coolant expansion (room temperature to operating temperature)

Displacement

The final engine selection tip for this post is regarding displacement.  The industry norm is to increase power density and extract more and more power form the same size engine.

This decreases footprint area and weight for a give power output – a real benefit to manufacturers of mobile and portable machinery.

Fuel consumption can sometimes improve when following this trend – but not always.

Backup power systems can benefit from increasing power density.  This means that less space is taken up in the building’s mechanical spaces.

For a distinct example, I currently see 4-cylinder engines producing the same power as their 6-cylinder counterparts (same bore and stroke).  This 33% increase in power density obviously drives up internal pressures and stresses.

In heavy-duty prime power applications you should look to avoid this trend.  Yes increasing power density is a benefit in some applications; but it is not desirable in other applications.

The saying in industry is “There’s no replacement for displacement”.

A larger displacement engine reduces cylinder firing pressures (BMEP – Base Mean Effective Pressure), and increases durability.

Also, unlike gasoline engines, an increase in diesel engine displacement does not directly lead to an increase in fuel consumption.  In the diesel engine, only the amount of fuel required to take up the load is injected.

In this post I have looked at six major engine selection tips that you should use when purchasing a generator-set.

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