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LO 11-2 Identify and explain the purposes and functions of the cooling system

1. Purposes and Functions of the Cooling System

1.3. Calculating Thermal Efficiency

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Calculating Thermal Efficiency
In today’s engines, more than half of the heat energy produced from combustion is wasted through heat radiated from engine surfaces and dissipated from the engine through the cooling system and exhaust gases. The measure of an engine’s ability to convert fuel’s energy content into mechanical force is called thermal efficiency Figure 5. Thermal efficiency is measured as a percentage of the potential energy content in fuel compared with the engine’s ability to convert the fuel energy into mechanical energy. A simplified way of stating thermal efficiency would be to compare the mechanical energy available at the flywheel to the cost of fuel. If $100 of fuel is burned by an engine and only $40 of mechanical energy is produced at the flywheel, the engine would have 40% thermal efficiency. Consider another example: In 2010, the best on-highway, HD diesel engines operated at approximately 45% thermal efficiency, meaning 45% of the fuel energy released during combustion was converted into mechanical energy and made available at the flywheel. The remaining energy was absorbed by the cooling system (30–35%), emitted by the exhaust system (20–25%), or radiated to the atmosphere through hot engine parts (5%). Friction and the energy required to turn accessories and move gases into and out of the engine accounted for the remaining losses in thermal efficiency.

   Fig11_5

FIGURE 11-5: Thermal efficiency is highest in a diesel engine. Newer diesels convert as much as 45–50% of their heat into force. Since 2016, diesel engines have been expected to meet phased-in targets for GHG emissions standards but will eventually need to achieve more than 60% thermal efficiency.

 

Larger, slow-speed, industrial diesel engines used on ships and for electric power generation can operate at more than 50% thermal efficiency. GHG emissions standards for HD diesels increase engine efficiency beyond this by requiring a 17% increase in thermal efficiency for diesel engines by 2025 from the 2010 base levels. This stretch goal of 17% is not set because diesels are inefficient. The thermal efficiency of today’s diesel engines is almost double that of spark ignition (SI) gasoline engines, which typically operate at up to approximately 25–30% thermal efficiency. Higher compression ratios, turbocharging, the absence of a throttle valve, and other mechanical factors also help diesels achieve better thermal efficiency. Recapturing waste heat energy that would normally be lost can potentially increase the thermal efficiency of a diesel engine by a significant amount. Downspeeding of engines, when the engine operates at lower revolutions per minute (rpm) at cruising speeds than traditional engines, is another strategy to improve thermal efficiency. With fewer combustion events for each mile or kilometre traveled, less heat is ultimately transferred to the cooling system.

 

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