Gasoline engine efficiency is lower than diesel engine efficiency, but modern gasoline engines have improved dramatically due to technologies like direct injection, turbocharging, variable valve timing, and high compression ratios.

Below is a deep, engineering-level breakdown of how gasoline engines work, why their efficiency is limited, and how engineers improve it.

1. Thermodynamic Basis: The Otto Cycle

Most gasoline engines operate on the Otto cycle, characterized by:

Constant-volume heat addition (spark ignition)
Compression of an air–fuel mixture
Rapid combustion across a homogeneous charge

The theoretical thermal efficiency of the Otto cycle is:

Where:

( \eta ) = ideal thermal efficiency
( r ) = compression ratio
( \gamma ) = ratio of specific heats (~1.35–1.4)

Typical compression ratios:

Old gasoline engines: 8:1 to 10:1
Modern engines: 10:1 to 14:1 (Thanks to direct injection + knock control)

Result:

Higher compression → higher efficiency
But gasoline engines are limited by knock, forcing them to run lower compression than diesels.

2. Knock Limits Efficiency in Gasoline Engines

Knock (autoignition of the air-fuel mixture before the spark fires) is the primary factor limiting gasoline efficiency.

Gasoline engines compress a premixed air-fuel charge, so if compression becomes too high, the mixture detonates.

Effects of knock:

Engine damage
Lower compression ratio limitations
Less aggressive ignition timing
Richer mixtures at high load

This reduces potential efficiency relative to diesel engines, which intentionally operate with compression ignition and control combustion using injection rather than spark.

3. Throttling Losses Reduce Efficiency Significantly

Gasoline engines control power by restricting air using a throttle plate.

At low load:

The throttle is partially closed
A vacuum forms in the intake manifold
Pistons waste energy pulling air past the restriction

This “pumping loss” is one of the biggest inefficiencies:

Bad in city driving
Significant at idle and low rpm

Diesel engines avoid this problem entirely.

4. Stoichiometric Combustion Requirement

Gasoline engines must run close to the stoichiometric ratio:

14.7:1 (air : fuel)

Why?

Catalytic converters require this mixture to reduce NOx, CO, and HC emissions.
Running lean (excess air) dramatically increases NOx emissions.

Diesels run very lean → more efficient
Gasoline engines run stoichiometric → lower efficiency

Stoichiometric burn:

Higher temperatures
Higher heat loss
More energy lost to coolant, exhaust, and cylinder walls

5. High Combustion Temperatures Increase Heat Loss

Premixed gasoline burns fast and hot:

Peak flame temperatures exceed 2,200°C
More heat transferred to cylinder walls and exhaust
Greater thermal conduction + radiation losses

Hotter combustion ≠ better efficiency.
It means more energy leaves the engine without doing useful work.

6. High RPM Operation = More Friction Loss

Gasoline engines rev higher than diesels:

Typical peak power: 5,500–7,000 rpm
Some performance engines: 8,000–9,000 rpm

High RPM increases:

Ring friction
Bearing losses
Valve-train friction
Oil pumping losses

So despite high power output, gasoline engines may be less fuel-efficient because a large portion of work counters internal friction.

7. Advantages That Improve Gasoline Efficiency

Despite limitations, modern gasoline engines achieve impressive efficiency (up to 40% in advanced engines like Toyota’s Dynamic Force or Mazda SkyActiv-G).

Here’s how:

7.1 Direct Injection

Allows:

Cooling effect of fuel evaporation inside cylinder
Higher compression ratios (knock resistance)
Stratified combustion at light loads
More precise fuel control

This alone can increase efficiency by ~5–10%.

7.2 High Compression Ratios (Up to 14:1)

New engines like Mazda SkyActiv-G use:

Special pistons
Long 4-2-1 exhaust manifolds
Very precise timing

This allows near-diesel-level compression without knock.

Higher compression → higher thermal efficiency.

7.3 Turbocharging

Turbocharging:

Recovers exhaust energy
Increases volumetric efficiency
Allows downsizing (smaller engine, same power)
Enables Atkinson-cycle-like efficiencies when cruising

But turbos can cause knock, requiring careful boost control and intercooling.

7.4 Variable Valve Timing and Lift (VVT/VVL)

Technologies like:

VVT
VVTL
VTEC
Valvetronic

Improve efficiency by:

Reducing pumping losses
Optimizing airflow for load and rpm
Enabling Miller or Atkinson cycle operation
Improving combustion stability

7.5 Atkinson/Miller Cycles

Hybrid engines and some efficient gasoline engines use:

Late intake valve closing
Larger expansion ratio than compression ratio

This increases:

Expansion work
Efficiency

At the cost of reduced torque (which hybrids offset with electric motors).

7.6 Exhaust Gas Recirculation (EGR)

Recirculating cooled exhaust gas:

Lowers combustion temperature
Reduces knock
Allows higher compression or earlier spark timing
Reduces pumping losses

EGR improves part-load efficiency, especially in highway cruising.

8. Real-World Gasoline Engine Efficiency Numbers

Typical gasoline engines:

25–30% thermal efficiency

High-end modern engines (e.g., Toyota Dynamic Force, Honda Earth Dreams, Mazda SkyActiv-G):

35–40% efficiency

Hybrid-optimized Atkinson-cycle engines:

41–43% efficiency (one of the best in the world for gasoline)

Even these top engines still generally trail diesel engines by 5–10% in most driving conditions.

9. Summary: Why Gasoline Engines Are Less Efficient

Limitation	Reason
Knock-limited	Lower compression ratios required
Throttle losses	Pumping losses at low load
Stoichiometric requirement	Limited ability to run lean
High combustion temperature	More heat loss
Higher RPM operation	More friction
Premixed combustion	Less flexible timing control