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5.2 Internal Combustion Engines

Technology Overview

Internal combustion engines date back to the mid- nineteenth century. Nikolaus A. Otto invented the four-stroke engine in 1876 and Rudolf Diesel patented the diesel engine in 1892. The diesel engine has a theoretical efficiency limit of 75.6% while modern compression-ignition systems have a fuel efficiency limit of up to 40%. These two technologies offer different solutions to render energy from a combustion engine. A normal gasoline engine mixes gas and air using a carburetor or fuel injection, compresses the air and then uses a spark to detonate the mixture. This process is known as the Otto cycle. A diesel engine operates slightly differently using compression technology. Instead of an air fuel mixture, a diesel engine takes in air, compresses it, and then injects the fuel. The heat of the compressed air is enough to ignite the fuel.

For the power range we are concerned with, both types of engines are appropriate. However, the generator set up (genset) and turnkey costs of diesel engines are about half the cost of gas engines. Hence, we are only considering diesel engines in the comparison. Diesel engines can be classified into three different categories that have been optimized for certain size power generation installations:

Low speed (200-400 rpm) - 100MW

Medium speed (800-1200 rpm) - 600kW to 7000kW

High speed (1500-3600 rpm) - 200 to 800kW

The target installation size dictates the engine speed level and fuel type. The medium- and high-speed diesel ICEs offer existing commercial solutions within our scope [12].

Technology Status

ICEs Ranked 1st

ICEs as the fundamental prime mover are the oldest type of implemented distributed generation system. Having existed in the commercial industry for nearly 100 years, they have evolved significantly from the original ICEs. Diesel engines are in use in hundreds of thousands of trucks worldwide and have been for many years. The compact and tailored designs make them the preferred backup power generation for most major corporations worldwide. Diesel engines have pervasively penetrated the world market. Due to this penetration and the relative age of diesel engines, they have become familiar to many mechanics and engineers. Diesel engines also possess a relatively simple design for maintenance purposes.   Age, ease of maintenance, and technical familiarity create a very highly reliable machine.

When compared to the theoretical efficiencies and the emissions efficiencies of other DG technologies, diesel engines must be optimized to maintain their current market stronghold.   Recent developments in small diesel engines indicate a promising future. The small engine market has traditional been ignored by the power industry. It is more difficult to emissions' optimize small units than large units. At Cummins Diesel, their popular small truck engines feature 'quiet diesel' with more precise full injections techniques that will undoubtedly carry over to larger machines [3]. Other automotive manufactures have developed engines that can be classified as 'ultra low emissions vehicle' horsepower range engines [4].   These engines present the power industry with potential future diesel low-emission DG technologies.

We have conferred the first place rank to ICEs due to their proven history, their wide market penetration, the ease of repair to these systems, and their current competitive status with other DG technologies.

Environmental and Noise Considerations

ICEs Ranked 4th

Despite the diesel engine's long history in the backup and uninterruptable power supply generator set up (UPS genset) market, the diesel engine design did not have emissions reduction as a priority. End users have accepted excessively high NOx and SOx emissions as the norm. However, now our (the U.S.) government restricts most operating times to less than 150 hours per year because of environmental concerns [10]. In addition to the environment considerations, most sub 5MW diesel gensets lag behind automotive engine developments and create quite a bit of noise. Diesel engines produce significant sources of environmental and noise pollution when compared to the other DG technologies discussed.

Device Cost

ICEs Ranked 1st

ICEs rank at the top of the list along with combustion turbines with a low $200-500 per kW range. ICEs have been able to achieve their low cost because of their long history and mass production. International price reduction agreements and contracts have been made with everything from individual parts manufacturers down to mining and materials vendors.

Figure 1. 1.75MW diesel gensets for DG applications [5].

Physical Size

ICEs Ranked 1st

Diesel engines have definitely benefited from technological advances of the automotive industry pertaining to size reduction. In order for high power engines to be able to fit under the hood of a 16-wheeler, higher power density was demanded. As a result, the power density of a diesel engine is .6 cubic feet per kW. Hence, diesel engine generators can be installed in more size restrictive locations than the other DG technologies. Figure 1 illustrates 1.75MW trailer sized Caterpillar gensets used in a DG application for peak shaving. In this form, the diesel engine offers portability and convenience. However, where the footprint is more of a concern, the serial nature of ICEs increases the length and width of the unit. Cursory examinations of product data sheets illustrate the microturbine's small footprint area.

Intended Application

ICEs Ranked 2nd

ICEs are very versatile in scaling. They can be built and are available from 1kW to more than 25MW for various duty cycles and load patterns [1]. The scope of this paper defines a 5MW maximum where many commercial and proven ICEs are available. Caterpillar currently has off-the-shelf solutions in this range as illustrated in Figure 2. Most ICEs are being used in backup power and peak shaving applications. Diesel engines will fit the intended DG application perfectly provided greenhouse gas emissions can be reduced to levels where they are operated year round.

Potential for Improvement

ICEs Ranked 4th

The long successful life of ICEs is not directly responsible for a low ranking in this category. Though diesel engines have current fuel efficiencies of near 40%, no significant improvements are expected in the near future. Noise reduction techniques are being researched and emissions levels are being improved by the development of a new type of clean diesel fuel. Even though reductions of 12% in NOx emissions and 24% in particulate matter emissions have been realized, a definitive solution for large-scale units may still be years away [6]. For Diesel engines to be a viable widespread DG technology, the greenhouse gas emissions must be significantly reduced.


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