The Otto and Diesel Cycles and How they are Applied for Competitive, Efficient Power Generation

The summer continuing education course, “Introduction to Thermal Power Plants” is a comprehensive and intense 3-day course where we present classroom instruction combined with tours of five power generating facilities. These facilities include utility scale power generation plants up to 850 MW and as small as the Williamson Energy Island where our largest single generator is 500 kW. The overall intention of the course is to show how heat energy is used to generate electricity and the various efficiencies of each type of heat engine.

Figure 1, above – Marcus Hook Energy Center is one of five power plants that will be visited during the course. This is a rare opportunity to see the inner workings of so many power facilities in one course.

I’d like more information about Williamson’s 2018 Introduction to Thermal Power Plants course!

The thermal power plants and the prime movers we will study are:

  • Exelon’s Eddystone Station, two 380,000 kW oil and natural gas-fueled peaking units
  • Dynegy’s Liberty Station, a 550,000 kW gas turbine, combined cycle unit, natural gas fueled
  • Marcus Hook Energy Center, 790,000 kW gas turbine combined cycle
  • Logan Clean Coal Plant, 225,000 kW coal-fueled plant
  • Williamson College of the Trades’ Energy Island, combined heat and power facility

Each of the four utility scale plants that will be toured dwarf the generating equipment at Williamson. However, although small, the steam turbines and CAT natural gas engine which uses the Otto cycle, are very efficient and in fact when natural gas prices are low, can generate power competitively with large central stations.

Thermal power generation from reciprocating engines has come a long way since Nikolaus Otto created the first internal-combustion engine that used the 4-stroke cycle in 1862. Otto’s invention was intended to provide an alternative power source to Watt’s steam engine invented about a century earlier. Rudolf Diesel filed the patent for the compression ignition diesel engine in 1892. So, the first hundred years of the Industrial Revolution included steam and internal combustion heat engines. The first internal combustion engines reached efficiency levels of about 10-20%. Now as you will see Otto and diesel cycles have reached central station plant efficiencies.

Here is a bullet list of seven steps in efficiency gains of modern heat engines and thermal power plants. For example:

  • Most efficient gas turbine combined cycle power plant, 63%[1] (Guinness World record by Toshiba, Jan 2018; GE previously held the record at 62% thermal efficiency)[2]
  • Most efficient ultra-supercritical coal plant, about 42% thermal efficiency (AEP Turk Plant)[3]
  • Most efficient diesel engine, 48%, Guinness World record by Wartsila Marine Engines of Finland [4]
  • Most efficient gasoline engine, 38% by Toyota [5]
  • Caterpillar 500 kW reciprocating natural gas engine as installed at Williamson, 37% efficiency [6]
  • 250 kW diesel installed at Williamson about 32-37% efficiency
  • Combined heat and power at the Williamson Energy Island, CHP = 75% thermal efficiency [9]

Williamson’s Energy Island uses both diesel and Otto cycle reciprocating engines as prime movers. The efficiency of the 500 kW SEG (Standby Electric Generator) approaches 37% at the most efficient load point. Of course, all of the peak efficiencies listed above are during ideal test conditions and at the most efficient loading. As an experienced field engineer who was charged with the responsibility of proving by testing the guaranteed performance from new installations, I completely understand the terms “Designed for and Guaranteed for.” The listed efficiencies above are at the best approach to design operating conditions.

The best overall energy use efficiency is with CHP (Combined Heat and Power). Williamson’s Energy Island is an example of the use of CHP where we use steam for power generation to two steam turbines and the exhaust heat is then recovered for campus heating in the winter. Later, the natural gas engine water jacket heating and exhaust heat recovery is planned to be added to the thermal storage system and used to heat campus buildings by circulating hot water. The campus has always used CHP since 1890 using steam turbine exhaust. The Wanamaker buildings have been heated with circulating hot water since 1959 when they went into operation. We hope to convert more buildings to use circulating hot water in the future. Why? To increase the steam turbine steam flow and the resultant CHP electricity generation. The huge efficiency of CHP is gained by using the natural gas for power generation first and then capturing the exhaust steam, reciprocating engine exhaust and water jacket heat. Heat that was once called “waste heat.” Through CHP the extremely high thermal efficiency numbers can be attained. See the figure below which is from a U.S. Department of Energy “Better Buildings” publication.


Summary on the Economics of Thermal Power Generation

In this short space, a lot was covered on thermal efficiency. To be fair and practical, the economics of power generation need to be considered as the ultimate consideration of economic dispatch of electric power. Let’s take an example of comparing a 48% efficient diesel generator with a 42% ultra-supercritical coal plant. At first glance, the diesel looks attractive as being much more efficient than the coal plant. When fuel costs per million Btu’s are compared though, the coal plant wins.

The fuel cost for a coal plant generation at a 42% thermal efficiency is about $0.014/kWh or $14.1/MWH. (Production cost from low cost coal is about $25/MWH when all O&M and FGD costs are considered, more below.)

The fuel cost for power generation by a 48% efficient diesel cycle would be $0.12/kWh or $120/MWH. Although a higher thermal efficiency, the production cost of generating power by diesel is more than five times as expensive as using coal in this example.

The above illustration is based on coal fuel at $1.74/million Btu’s (based on delivered coal cost of $40.00/ton and 11,500 Btu’s/pound) and diesel Fuel $ 17.00/million Btu’s based on $2.50/gallon and 146,000 Btu’s/gallon.)

The cost of power generation as stated above is for the fuel cost component only. The total cost of generation requires additional components such as initial capital cost, maintenance, water and flue gas desulfurization (FGD) and ammonia for SCR (Selective Catalytic Reactor) treatment chemicals for the coal plant and operational costs. These must be added. However, fuel cost alone is about 85% of the generation cost for diesel generators and the fuel cost component of a large coal plant is about 75-80% of the production cost of electricity.

Heat engines convert the potential chemical heat energy of a fuel into mechanical power to turn a generator. Therefore, the cost of each fuel must be considered for the most cost-effective generation. The recent approximate costs of common fuels, expressed in dollars per million Btu’s (British Thermal Unit) are:

  • Natural gas about $3.00/million Btu
  • Coal about $2.00/million Btu
  • Diesel fuel about $17.00/million Btu

So, high efficiency is very important. However, the cost of fuel is also important when considering the production cost of electricity. This is why it is important for America to have “Balanced Generation Portfolio” using the diversity of all fuels. Thermal power generation provides about 85% of America’s electric power generation. The fuel cost is the single largest cost component for thermal power generation as shown in the comparisons of high efficiency diesel power generation compared to slightly less efficient clean coal power generation.

For further reading on the importance of fuel diversity and power costs, please refer to references 8,11,12 below. The NETL (11)Infographic shows the importance of large coal and nuclear plants during the first week of January this year when unseasonably cold weather was experienced. Coal and nuclear provided over 58% of the electric power needed for the grid during this critical time. Williamson’s energy island also has flexibility to use diesel fuel during times such as this when natural gas availability may be limited due to high demand and pipeline choking.

The reference #12 by the National Mining Association shows the power costs in the U.S.A. and the percentage of coal generation of each state.

One of our presenters, Kevin Hatch is from the PJM Interconnection. During our course, Kevin will provide a presentation on how the PJM determines which power generators are selected by lowest production cost and the impact that Renewable power is making on the grid. Stacy Starr will provide a presentation on the economics and reliability of solar power as installed at Williamson.

Energy use in thermal power plants has come a long way since James Watt applied the steam engine for practical purposes in the late 1700’s! If you would like to learn more on thermal power generation, consider participating in our summer course. Much of the above will be explained and demonstrated.

Richard F. (Dick) Storm, PE, CEM
Williamson class of 6W2

Registration is now open for Williamson’s 2018 Introduction to Thermal Power Plants course!


  1. Toshiba Energy Systems
  2. GE Gas Turbine, Combined Cycle Plant designed for up to 64% Thermal Efficiency
  3. POWER Magazine, April 2017 World’s Most Efficient Coal Plants
  4. Wartsila 46DF Marine Engine
  5. Toyota Gasoline Engine Reaches 38% Thermal Efficiency
  6. Caterpillar Natural Gas Engine G3412C Gas Engine Technical Data
  7. EIA (Energy Information Agency) Electricity Cost of Production Chart
  8. U.S. Chamber of Commerce Publication, “Here is Where Your State Stacks up in Electricity Prices”
  9. U.S. Department of Energy Combined Heat and Power “Better Buildings” Website
  10. Engineering Toolbox for Fuel Heating Values and analyses
  11. The Importance of Power Generation by Coal and Nuclear Plants during January 2018, by NETL/DOE.
  12. National Mining Association cost of electricity by each state and the percentage of coal generation in each state.
2018-04-18T13:50:14-04:00 April 17th, 2018|