Diesel heaters are known for their efficiency due to several key factors:

1. High Energy Density of Diesel: Diesel fuel has a higher energy density than other fuels like gasoline. This means diesel contains more energy per liter, making diesel heaters capable of producing more heat from a smaller amount of fuel.

2. Efficient Combustion Process: Modern diesel heaters use advanced combustion processes. They often have precise fuel injection systems and can achieve a more complete combustion of the fuel, thus extracting more heat.

3. Good Thermal Efficiency: Diesel heaters convert a high percentage of the fuel's energy into heat. This is partly due to the higher boiling point and combustion temperature of diesel fuel, which leads to a more efficient conversion of energy into usable heat.

4. Durability and Reliability: Diesel heaters are known for their durability. They can operate efficiently in harsh conditions and require less maintenance compared to other types of heaters, which contributes to overall efficiency in terms of both energy usage and cost.

5. Variable Heat Output: Many diesel heaters offer adjustable heat settings. This allows users to select the exact amount of heat needed, preventing energy wastage.

6. Direct Heating System: Diesel heaters typically work by directly heating the space without the need for a heat exchanger. This direct method is more efficient as it minimizes heat loss.

7. Low Emissions: Newer models are designed to burn diesel cleanly, reducing waste and emissions. This makes them not only more efficient but also more environmentally friendly compared to older models.

It's important to note, however, that the efficiency of a diesel heater can depend on its design, maintenance, and the quality of diesel used. Regular servicing and using clean, high-quality diesel can help maintain the heater's efficiency.