Bringing in the new

Siàƒ¢n Green

With distributed generation on the increase, Caterpillar aims to grow its share of the market with its new G3500 Series of natural gas engines.

“This new generator set fills an emerging need in the distributed generation market,” said Michael A. Devine, gas product marketing manager with Caterpillar Inc. “Distributed generation is growing rapidly as utilities see its potential as a quick, low-risk way to bolster power supplies.”

Devine was speaking at the launch of Caterpillar’s new G3500 Series of natural gas fuelled generator sets at Power-Gen International in Orlando, USA. Based on existing, proven Caterpillar technology and incorporating a number of new design enhancements, the G3500 Series is designed to deliver low-cost electricity in extended-duty distributed generation applications.

Figure 1. The new G3500 Series is based on the existing Cat 3500 technology base
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“End users are gravitating towards distributed generation as they see opportunities to increase their power reliability, increase operating efficiency and turn increasingly competitive electric power markets to their advantage,” continued Devine. “These trends are driving demand for generating systems that combine low emissions with attractive operating costs in applications involving extended annual hours of service.”

The new G3500 Series is, says Cat, designed to meet these needs. The generator sets are driven by Cat advanced lean-burn gas reciprocating engines with ratings up to 2.1 MW. They can deliver up to 43.5 per cent mechanical efficiency with maintenance costs equal to or lower than those of competing systems, says Cat. Standard NOx emissions are 250 and 500 mg/Nm3, while power density is high.

Natural gas drivers

The development of a new series of natural gas engines is in line with the projection that the use of natural gas for distributed generation world-wide is projected to double in the next 20 years, according to the US Department of Energy. In the US, the share of natural gas in power generation is expected to increase from 15 per cent in 1999 to 32 per cent in 2020. This trend, says Cat, will involve the increased use of natural gas-fuelled reciprocating generator sets.

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The growth in demand for natural gas for power generation will be driven largely by air quality regulations. “As concerns rise about air pollution and global climate change, governments are steadily tightening emissions limits on nitrous oxides and other pollutants,” said Devine.

The development of the distributed generation market is aligned with these trends. “Distributed generation fuelled by natural gas has moved squarely into the mainstream of energy planning for utilities and end users,” commented Devine. “This has happened because political, economic and market forces are changing the dynamics of how electric power is produced, sold and delivered.”

ARES technology

The new G3500 Series has been designed to suit a wide range of distributed generation applications. Built on the heavy-duty diesel-based 3500 technology base, the new series incorporates several advanced features developed under the US Department of Energy’s Advanced Reciprocating Engine Systems (ARES) programme.

The ARES programme began in 1998 and its objective is to develop a class of internal combustion engines that meet stringent requirements for NOx, SOx, CO2 and other emissions whether using natural gas, propane, biofuels or clean sulphur free oil, and to make the engines available at a competitive price. The programme involves a consortium of engine manufacturers and by 2010 aims to produce natural gas reciprocating engines with 50 per cent mechanical efficiency and less than 0.01 g/bhp-hr (9 ppm) NOx emissions.

Figure 2. The control module is mounted in a diecast aluminium housing
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According to Devine, features incorporated into the G3500 Series and its first unit ” the G3520C ” are a direct result of ARES. New features include an improved cylinder head design, larger, more robust air intake and exhaust systems compared to earlier models, and a new fuel valve and air/fuel control strategy.

The new G3500 Series is based on the 3500 technology base, which was first introduced in the mid-1980s and of which there are now some 9500 units in operation around the world. Caterpillar says that it has “kept the best” of this technology in the new G3500 Series, including major components such as the block, crankshaft, rods and camshaft, all of which are design for large loads. These components have demonstrated long life capability and will contribute towards lowering maintenance costs in the new series.

Advanced features

The new G3500 Series is the only generator set in the 2 MW class that uses an open combustion chamber design. This design eliminates the need for a pre-combustion chamber, thus increasing the simplicity of the design and reducing initial capital cost as well as maintenance costs. It also allows a certain degree of flexibility on fuel quality. The spark plugs can also be removed without disturbing other parts of the engine, improving ease of maintenance.

The open chamber design allows the use of low pressure gas (35-350 mb). The engine can also handle a variety of fuels with methane numbers down to 70 without power derate. Fuels with a low methane number can be harder to burn and can cause detonation, which can lead to damage in the cylinder. To prevent this, detonation sensors monitor each cylinder and automatically change the timing of the engine to prevent detonation. In extreme cases, the sensors will shut down the engine to avoid damage.

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The engine also incorporates a low-swirl piston design. This, says Devine, gives more complete combustion than high-swirl designs. It also gives greater control over the flame propagation process, helping to improve the efficiency and emissions level of the engine.

Detonation sensors

Fuel intake into the engine is controlled by an electronic fuel control valve that is of proprietary design and is superior to valves used in the past, according to Devine. The valve receives a control signal from the engine control system to ensure proper fuel flow delivery. In addition, the closed loop air/fuel ratio control system monitors charge air density under all ambient and load conditions to tightly control NOx emissions.

The new G3500 Series uses an ADEM III-based digital microprocessor control unit, which automatically regulates engine governing, air/fuel ratio and ignition to optimize efficiency, fuel economy and emissions.

The ADEM III control system allows the engine to respond to block loading and unloading without significant changes in speed, thus minimizing frequency and voltage fluctuations. When a kilowatt sensor detects a block load, control software momentarily adjusts the air/fuel mixture to enable the generator set to respond smoothly. When a block load drops offline, the control system compensates by bypassing compressed air and fuel from the aftercooler to the inlet side of the turbocharger. This reduces the air/fuel charge to the cylinders and improves power quality during a load drop transient.

The control module is SCADA-compatible and sealed in a shock-mounted, diecast aluminium housing. It is insulated against electrical noise and is accurate at temperatures ranging from -40à‚°C to 120à‚°C. Primary and secondary modules are provided for redundancy. The unit also has remote monitoring capabilities.


The new G3500 Series gensets are driven by gas engines in 12, 16 and 20 cylinder configurations, operating at 1500 r/min (50 Hz), 1200 r/min (60 Hz) or 1800 r/min (60 Hz), with ratings up to 2.1 MW. The gensets can be packaged either as an open genset or as a combined heat and power (CHP) genset package.

The open genset package will be available in February 2003, and has options such as remote radiators. The CHP genset package is designed for jacket water and/or exhaust heat recovery and uses electronic water pumps.

According to Cat, the new G3500 Series is ideally suited to distributed generation and CHP applications running 2000-3000 hours per year. In such applications, gas engines perform well compared to diesel recips, as well as other distributed technologies, in terms of maintenance and operating costs. The policy environment in Europe provides particular opportunities for this series in district heating and CHP applications, says Devine.

Cat has already installed nine new G3500 units, including one in Denmark in a greenhouse CHP application. This unit has been operating since the beginning of November 2001 and the company is monitoring the unit to ensure that maintenance, service and operational targets are being met.

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