The Differences Between Petrol and Diesel Engines

Summary

This video explores the key differences between petrol and diesel engines, from their ignition methods to their efficiency. While both engines follow the same four-stroke cycle, diesel engines use higher compression ratios, allowing ignition without a spark plug, making them more fuel-efficient. Diesel engines also lack a throttle body, using engine braking differently than petrol engines. With more energy-dense fuel, higher torque at lower RPMs, and turbocharging, diesel engines are designed for efficiency. The video delves into the science behind these differences, explaining why diesel engines offer better performance and fuel economy, particularly for torque and energy output.

Takeaways

• 😀 Diesel engines use **compression ignition**, while petrol engines use a **spark plug** to ignite the fuel-air mixture.
• 😀 Diesel engines have a **higher compression ratio** (e.g., 16.2:1 in VW Golf TDI), resulting in more power and efficiency.
• 😀 Petrol engines typically have a **lower compression ratio** (e.g., 9.6:1 in VW Golf TSI) because they rely on spark ignition.
• 😀 **Diesel fuel** contains about **15% more energy** by volume than petrol, making diesel engines more fuel-efficient.
• 😀 Diesel engines have no **throttle body**; they control power by injecting more fuel directly, unlike petrol engines that use a throttle to control air intake.
• 😀 Diesel engines produce more **torque at lower RPM**, allowing them to use less fuel to move the vehicle.
• 😀 Diesel engines are more **fuel-efficient** due to their higher compression and longer strokes.
• 😀 **Turbocharging** is common in diesel engines, allowing them to force more air into the cylinder, improving energy efficiency.
• 😀 In **engine braking**, petrol engines rely on a **vacuum** created by a closed throttle, while diesel engines rely on **compressed air escaping** during the compression stroke.
• 😀 Modern diesel engines have become **more refined**, with less reliance on throttle bodies and better fuel delivery systems.
• 😀 The **Audi R8 V10 Plus** with its 5.2L V10 engine can rev up to 8700 RPM, offering high performance, while demonstrating the difference between high-revving petrol engines and diesel engines that focus more on low-end torque.

Q & A

What is the basic operational difference between petrol and diesel engines?

The main difference between petrol and diesel engines is how they ignite the air and fuel mixture. Diesel engines rely on compression to ignite the mixture, while petrol engines use a spark plug to ignite the mixture.

How does the compression ratio differ between petrol and diesel engines?

Diesel engines have a higher compression ratio than petrol engines. For example, a 2015 VW Golf TDI has a compression ratio of 16.2:1, while a VW Golf TSI has a ratio of 9.6:1. This higher compression in diesel engines ensures that the air and fuel mixture is compressed enough to ignite without a spark.

What is a compression ratio and why is it important?

A compression ratio is the ratio of the maximum to minimum volume in a cylinder. It affects the efficiency and power of the engine. A higher compression ratio, like in diesel engines, results in higher temperature and pressure, which is essential for self-ignition of the fuel mixture.

Why don't diesel engines have a throttle body?

Diesel engines do not have a throttle body because they rely on adjusting the amount of fuel injected to control power. In petrol engines, the throttle body controls airflow into the engine, and more air means more fuel for more power.

How does engine braking differ between diesel and petrol engines?

In petrol engines, engine braking occurs when the throttle body closes, creating a vacuum that slows the vehicle. In diesel engines, engine braking happens during the compression stroke as pressurized air is allowed to escape through the exhaust valve.

Why are diesel engines more fuel-efficient than petrol engines?

Diesel engines are more fuel-efficient because diesel fuel contains about 15% more energy per liter than petrol. Additionally, diesel engines use higher compression and typically feature turbocharging, which helps them produce more torque at lower RPMs and require less fuel to move the vehicle.

How does the fuel composition of diesel contribute to its efficiency?

Diesel fuel contains more long-chain hydrocarbons, meaning it has more energy compared to petrol. This higher energy density allows diesel engines to be more fuel-efficient.

What role does turbocharging play in a diesel engine's performance?

Turbocharging plays a key role in a diesel engine’s performance by feeding compressed air into the cylinder. This helps the piston descend into the cylinder more efficiently, saving energy and improving overall fuel economy.

How does the higher compression in diesel engines impact torque production?

Higher compression in diesel engines allows them to produce more torque at lower RPMs compared to petrol engines. This means that diesel engines are better at moving heavy loads with less fuel consumption.

What is the significance of self-ignition temperature in diesel engines?

The self-ignition temperature is the temperature at which the air-fuel mixture in an engine will ignite purely due to compression, without the need for a spark plug. Diesel engines rely on this principle, which is why they have higher compression ratios compared to petrol engines.