Exploring the Pros and Cons of Gas vs.Diesel Engines
The rise in fuel prices has led some to consider engine options for fleet vehicles. In deciding what engine to use in a fleet vehicle, several factors must be considered:
Vehicle function - for what job is the vehicle used?
Environment - where will the vehicle be used: highway, inner city, high or low idle?
Power equipment - what will the vehicle operate: drills, compressors, aerial devices, pumps?
The following is a brief comparative analysis of gasoline and diesel engines. In choosing a new engine, keep in mind that a comprehensive evaluation of both engines would entail considering such additional factors as fuel infrastructure facilities, driver and mechanic training, supply and demand, work methods, environmental applications, and fleet growth.
Purchase Cost and Fuel Consumption. The initial purchase cost of a gasoline engine is lower; however, its rate of fuel consumption is higher. Fuel is more widely available and winter fuel treatment is invisible.
Maintenance and Parts. Lighter than diesel units, gasoline engines require more service attention. However, developments in electronic ignition and fuel injection systems will lead to extended maintenance cycles. The intervals between overhauls generally are shorter with gas engines. Replacement parts cost less and are readily available.
Operations. A gasoline engine’s higher governed speeds allow a more flexible engine. With less engine torque, lighter vehicle weight components can be used to transmit power to the rear wheels. The low starting torque requires less battery capacity.
Gas engines have inefficient idling characteristics, but start well in cold weather.
Technology. Fuel injection and upgraded electronic ignition and trouble-code diagnostic systems reduce maintenance costs. However, when they fail or experience difficulties, these same systems require high replacement costs and sophisticated troubleshooting and training.
Purchase Cost and Fuel Consumption. Initial costs for a diesel engine can run as much as $3,000-$8,000 over gasoline engine cost for larger vehicles, $3,000-$5,000 more for 2002 engines, and $3,000-$5,000 more for 2007 engines.
Diesels have good fuel economy, achieving up to 100-percent better mileage than gasoline vehicles. Diesel injectors require clean fuel, free of water and dirt, and in 2007 are mandated to use low-sulfur fuel.
Number 2 diesel fuel requires blending with additives and/or Number 1 diesel fuel to prevent clouding or jelling in fall, winter, and spring seasons.
Maintenance and Parts. The higher engine torque of a diesel engine requires stronger, heavier components, including not only the engine block, but also such support components as tires, transmission, front suspension, axles, and drivelines.
The combustion system is free of electrical maintenance.
Scheduled maintenance for diesels is 50-percent less frequent than for gasoline engines. Diesels can go farther between overhaul intervals, provided drivers don’t abuse the trucks and mechanics do a respectable PM job.
Diesel engine replacement parts are expensive. Rebuilding can cost up to three times the cost of rebuilding a gasoline engine. In addition, part availability can be a problem, especially on foreign engines.
Operations. Lower governed speeds in diesel engines can lead to more engine lugging and can also mean lost productivity if a truck can’t keep up with traffic. Driver training and automatic transmissions will help lower this risk and increase efficiencies.
Heavier components mean that heavier frame rails, bracketing, and other parts are needed to support the heavier components, and this added weight means less mpg.
Diesels have better idling characteristics than gasoline engines and can idle four times as long on the same amount of fuel.
Cold weather starts for diesel engines require fuel additive assistance or block heaters, either of which could damage the engine.
With a high starting torque, diesels require either more battery capacity or air starters. There is greater opportunity to apply a diesel to the wrong sort of job or to use it incorrectly and be saddled with catastrophic engine failure.
Technology. Mechanics require specialized knowledge to work on diesel injection systems.
Diesels require systems for emissioncontrol, now met with turbocharging low-sulfur fuels, particulate traps, and exhaust gas recirculating technologies that cost more, require extra maintenance for heat-generated failures, and result in lower mpg.
Analyzing the Costs
In performing a basic cost analysis of moving from gasoline to diesel engines, meaningful indices must be measured. Costs are then funded with capital and operating dollars, though these can be limited due to internal company strategic and tactical planning activity.
Chart 1 illustrates a comparative cost analysis of a 2006 diesel truck versus a 2006 gasoline-powered truck. The costs include a settled-down fuel cost of $2.60 per gallon for diesel and $2.25 per gallon for gasoline, and a $5,000 price difference per truck with a one-year average maintenance cost extended out for an 84-month lifecycle. An estimated 12 months is needed to cover the additional price difference and the 72-month remaining lifecycle shows a $31,000 potential savings for a medium-size truck, for example, a gardener’s dump truck with a 3-5-yard capacity.
Of course, the higher the acquisition cost difference, the longer the return on investment. Also, most gasoline truck capacities stop at 24,000 lb. GCVW and a diesel is a minimum needed to work this weight.
If the cost of the cure is more than the cost of the disease, bear the disease.
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