Environmental concerns and issues related to diesel fuel storage have increased interest in bi-fuel engine products. The concept of bi-fuel engines is not new — Rudolf Diesel himself experimented with enriched combustion air mixtures in the early 1900s — and various methods of metering natural gas into diesel engine cylinders have been successfully proven in commercial operation. Most of these have relied on mechanical control systems. With modern microprocessor controls, a more precise system can be designed that maintains an optimal balance of diesel and natural gas fuels across the engine’s power range.
After years of development and testing, Generac Power Systems is now producing bi-fuel versions of its proven SD300 (300 kW) and SD375 (375 kW) diesel standby engines. The engines offer the power and reliability of a conventional diesel engine, but with several additional benefits associated with natural gas technology: significantly lower emissions profile, reduced on-site fuel storage, and extended run times. For example, a 300 kW diesel genset with a medium-sized 24-inch diesel tank consumes 20.5 gallons of fuel per hour at 80% load, giving it a theoretical run time of about 21 hours. The same bi-fuel genset uses about 2.4 gallons of diesel fuel per hour at 80% load, allowing it to operate more than 182 hours.
There are several proven methods for implementing bi-fuel operation. Generac’s approach, called combustion air gas integration, introduces the gaseous fuel with the intake combustion air prior to the turbocharger, taking advantage of the speed and response of the microprocessor system. This approach offers the broadest range of operation at the lowest cost, and is best suited for the standby generation market, according to Generac.
During initial start-up, the bi-fuel generator operates on 100% diesel fuel. After certain permissive criteria are satisfied, such as the engine coolant temperature reaching 160 F, the controller initiates bi-fuel operation. It meters in an increasing amount of natural gas with the intake air, and at the same time proportionately reduces the diesel fuel until it reaches the optimum bi-fuel ratio. Throughout the process, the controller continuously monitors a variety of engine and generator parameters, including intake air temperature, engine coolant temperature, intake manifold temperature and pressure, kW load, and engine speed.
Through extensive mapping of these variables and their effect upon engine performance, the microprocessor automatically adjusts the bi-fuel ratio and fine-tunes the mixture for optimum engine operation. Under typical operation, the optimum bi-fuel ratio is about 10% diesel and 90% natural gas.
The integration of the bi-fuel system does not derate the generating set’s output capability, since it is specifically tailored to operate within the design parameters of the engine. The generator load response and stability also remain the same.
The bi-fuel engine comes with built-in fuel redundancy. If the natural gas supply is interrupted for any reason, the controls will automatically direct the unit back to 100% diesel without interrupting operation. Emissions are significantly reduced compared to 100% diesel — approximately 30% lower NOx emissions and 50% lower smoke and particulate matter emissions.