You are looking for a new car (about 100 kW nominal power) with the best possible energy efficiency. Which of the engine options should you choose solely based on the engine efficiency?Image: Wikimedia commons, CC BY-SA 4.0.

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BlackTom AI · Accepted Answer

To determine which engine option offers the best energy efficiency for a car around 100 kW nominal power, several aspects of each technology must be considered.
Option a: Gas turbine. Gas turbines can deliver high power-to-weight ratios and operate efficiently at high turbine inlet temperatures, but their overall vehicle efficiency is often limited by poor part-load efficiency and high exhaust energy losses in many automotive configurations. They typically require a heavy and complex drivetrain to convert the shaft power efficiently, and throttling losses at varying vehicle speeds can reduce steady-state efficiency.
Option b: Gas engine. Internal combustion gas engines (spark-ignited or compression-ignited) can achieve reasonable thermodynamic efficiency but are constrained by finite temperature and pressure cycles, friction, and pumping losses. At typical automotive operating regimes with frequent stop-and-go, their brake thermal efficiency is generally lower than that of fuel cells, due to combustion irreversibilities and heat losses that cannot be recovered.
Option c: Fuel cell. Fuel cells convert chemical energy directly into electrical energy with fewer energy transformations than heat engines, leading to higher theoretical and practical efficiencies, especially when waste heat is effectively recovered in a combined heat and power context. In automotive applications, fuel cells combined with electric drivetrains can maintain high efficiency across a broad load range, and the absence of internal combustion reduces many parasitic losses associated with piston, valve, and exhaust flows. Although hydrogen or another fuel source is required, the electrical energy pathway tends to yield the best overall efficiency for steady propulsion at moderate power levels like ~100 kW.
Therefore, evaluating the energy conversion pathways, the fuel-cell option typically offers the highest efficiency in automotive applications for this power level, followed by other technologies that suffer from engine-level irreversibilities and losses. In summary, the fuel cell route is the most favorable for maximizing energy efficiency under the given constraint.

You are looking for a new car (about 100 kW nominal power) with the best possible energy efficiency. Which of the engine options should you choose solely based on the engine efficiency?Image: Wikimedia commons, CC BY-SA 4.0.

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You are looking for a new car (about 100 kW nominal power) with the best possible energy efficiency. Which of the engine options should you choose solely based on the engine efficiency?Image: Wikimedia commons, CC BY-SA 4.0.

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