Difference Between Diesel and Petrol Engines - Otto Cycle, Diesel Cycle, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:54 PM IST

Difference between petrol and diesel engines.

The Petrol engine works on Otto cycle but the Diesel engine working on Diesel cycle is the main difference between Petrol and Diesel engines. Petrol engines are also known as spark ignition engines. The major difference between Petrol and Diesel engines is that Petrol engines use the Otto cycle, while Diesel engines use the Diesel cycle. Other differences between these engines include their structure, types, and applications.

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Difference between petrol and diesel engines.
What happens if Petrol is used in a Diesel Engine and Diesel in a petrol engine?
Difference between Otto cycle and Diesel cycle

The type of fuel these engines consume is the primary criterion by which they are categorised. Normally, these engines operate on the heat transfer concept. A spark-ignited petrol engine with the same power output can be 40% less efficient than a diesel engine based on the combustion process and overall engine concept. Diesel fuel has a calorific value of roughly 45.5 MJ/kg, while petrol has a calorific value of 45.8 MJ/kg, indicating that diesel has a lower calorific value than petrol. Diesel fuel has a higher density than petrol and provides nearly 15% greater energy by volume.

Despite the fact that the diesel engine is heavier, the total efficiency of the petrol engine is still 20% lower than that of the diesel engine. The main distinction is that petrol engines utilise spark plugs to ignite the air-fuel mixture, whereas diesel engines rely on heavily compressed air without spark plugs. As a result, air in diesel engines is compressed to roughly 14 to 23 times its original volume, whereas air in petrol engines is crushed to a far lower ratio.

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Diesel and petrol cars provide oomph in various ways to drivers. Petrol engines are all about revolutions per minute, and higher rpm results in more power. You go faster when you upshift through the gearbox with a more sporty sensation. Diesel engines offer their oomph – torque that can propel you up a steep slope – at lower rpm, which is crucial for towing or hauling a load. Diesel engines are more fuel efficient and utilise 15% to 20% less fuel, resulting in lower operating expenses.

Diesel automobiles with more low-end torque are ideal for highway cruising since overtaking is simple and often without the need to downshift. However, because diesel technology is more expensive, they normally cost more than petrol automobiles. In comparison to petrol cars, servicing or repairing a significant problem on a diesel car may be slightly more expensive. Although diesel vehicles emit less CO₂, pre-BS6 diesel vehicles emit small soot particles that have been related to lung diseases such as asthma.

Diesel engines are slightly noisier than their petrol counterparts, although this has improved dramatically as OEMs have developed novel noise-control measures. With the windows up, you can scarcely tell which engine you're driving because modern diesels are so quiet. Petrol vehicles, on the other hand, are slightly less expensive to purchase and maintain.

They produce more CO₂ emissions than diesel, but they produce fewer other emissions. They are quieter than diesels, but they are also less efficient and consume more gasoline. Because petrol engines create less torque than diesel engines, they must be driven in lower gear more frequently, such as when overtaking - though some drivers like this style of driving.

In today's world, diesel and gasoline engines are frequently compared in terms of their environmental impact like emissions and noise. To name a few key environmental concerns we face today, we have air pollution, greenhouse gas emissions, waste management, increasing water shortages, falling groundwater tables, water pollution, biodiversity loss, land and soil degradation, and a decrease in the quality of life index.

There are perceived issues in addition to true environmental concerns. Some people believe that Diesel engines will be banned in a few big cities, or that the life of Diesel vehicles would be limited in cities. This conjecture raises the issue of resale value, and we all know that cost and resale value are important considerations in any car purchase.

Modern diesel engines have always been more fuel efficient, long-lasting, and capable of delivering more carrying power (torque) than gasoline engines. They typically emit fewer hazardous pollutants, however they did (in the past) produce more carbon in their exhaust than petrol engines. Modern diesel engines are extremely advanced machinery.

OEMs and suppliers collaborated to improve engine technology with breakthroughs in engine design, exhaust pollution control systems, the catalytic converter, and ‘AdBlue,' a chemical that helps cut emissions. Modern diesels are lighter, more efficient, and technologically advanced than their predecessors.

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What happens if Petrol is used in a Diesel Engine and Diesel in a petrol engine?

Petrol is significantly lighter than diesel and ignites at lower temperature. Filling up your diesel automobile with petrol will harm the fuel injection system and put tremendous strain on the engine. If petrol is used in a diesel engine then the lubricating qualities are diminished. This means that the gasoline pump in your diesel car may produce undesired metallic particles, which may cause your engine to deteriorate over time. Filling a diesel engine with gasoline can cause your vehicle to detonate in extreme situations due to unrestrained fuel ignition.

Diesel fuel has a much heavier viscosity and a much greater auto-ignition temperature than petrol. Because most diesel nozzles at gas stations are larger than the filler neck of most petrol vehicles, you're less likely to fill up your petrol engine with diesel fuel. Most insurance policies and warranties do not cover damage caused by using the wrong fuel, so it's in your best interest to make sure you're always using the right fuel for your vehicle.

The sooner you understand you've used the wrong type of fuel, the less likely you are to suffer serious damage. Once you've noticed your car's tank has been filled with the wrong fuel, pull over in a safe spot, turn off your engine, and contact roadside help. Your roadside assistance service should assist you in safely draining the improper fuel from your tank. Never try to syphon petrol from your vehicle since there's a chance you'll end up with leftover fuel, putting extra strain on your automobile and potentially damaging your engine.

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Difference between Otto cycle and Diesel cycle

A typical spark ignition piston engine is described by an Otto cycle (petrol cycle), which is an idealised thermodynamic cycle. It is the most frequent thermodynamic cycle seen in car engines. The Otto cycle describes what occurs to a gas mass as pressure, temperature, volume, heat addition, and heat removal are changed. The system refers to the mass of gas that is subjected to those changes. The fluid (gas) within the cylinder is defined as the system in this situation. It would also describe the system's impact on the environment by detailing the changes that take place inside the system.

By detailing the changes taking place in the system, it will also describe the effect of the system on the environment in reverse. In the case of Otto cycle, the effect will produce needed network from the system so as to propel an automobile and its occupants in the environment. The cycles are most commonly encountered in a car engine. Petrol is the fuel used in the Otto cycle. It's also known as the Isochoric process since heat is added in a constant volume method.

The Diesel cycle is a reciprocating internal combustion engine's combustion process. The heat generated by the compression of air in the combustion chamber ignites the fuel, which is subsequently injected into the combustion chamber. This is in contrast to the Otto cycle engine, which uses a spark plug to ignite the fuel-air mixture. Aircraft, vehicles, power generation, diesel-electric locomotives, and both surface and underwater ships employ diesel engines. Diesel fuel is the fuel used in the diesel cycle. Heat is added at constant pressure in the diesel cycle, which is also known as the Isobaric Process.

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Frequently Asked Questions (FAQs)

1. What is SI engine?

The SI engine in full form is spark-engine. The Spark Ignition (SI) Engine is a type of engine in which the spark generated by the spark plug ignites the fuel. It runs on petrol and follows the Otto cycle. The air-fuel combination is injected into the cylinder with the help of a carburettor in a spark ignition engine. Although the fuel is compressed, it has a low compression ratio. The spark generated by the spark plug ignites the fuel. SI engines are quieter and more vibration-free, and they are simple to start. They are light in weight and require less upkeep. They're commonly seen in light commercial vehicles including scooters, motorcycles, and automobiles.

2. What is a CI engine?

CI engine full form is, CI stands for compression ignition engine. The Compression Ignition (CI) Engine is a type of engine in which the heat of compressed air is used to ignite the fuel. It runs on diesel and follows the Diesel cycle. Only air enters the cylinder during the suction stroke in a compressed ignition engine. Because of the high ignition temperature of diesel fuel, it has a high compression ratio. The compressed air's heat ignites the gasoline. It generates higher power due to the high compression ratio. More hydrocarbons are produced as a result of incomplete combustion of the fuel, resulting in air pollution. The CI engines have an issue with noise and vibration. When compared to SI engines, the CI engine has a higher maintenance cost. They're commonly seen in heavy-duty vehicles including buses, trucks, trains, and ships.

3. Write a note on the compression ratio of petrol and diesel engines?

The 'Compression-Ratio' in diesel engines ranges from 18:1 to 23:1, depending on the engine design and construction. Petrol engines often have a lower compression ratio. As a result, the compression ratio of petrol engines ranges from 10:1 to 14:1. The air and gasoline are compressed in a 10:1 to 14:1 ratio in a petrol engine.

4. What is TDI?

TDI full form is, TDI stands for Turbocharged Direct Injection. The Volkswagen Group's turbocharged diesel engines with an intercooler in addition to the turbo compressor are referred to as TDI. TDI engines are found in Audi, Volkswagen, and Skoda automobiles, and also as boat engines from Volkswagen Marine and industrial engines from Volkswagen Industrial Motor.

5. Write about fuel injection on diesel engines?

Fuel injection is the process of injecting fuel into an internal combustion engine, most often a car engine, using an injector. The diesel engine's fuel injection system is at its fundamental core. The system forces the fuel into air which has been compressed to high pressure in the combustion chamber by pressurising and injecting it. Fuel injection pump that pressurised fuel to high pressure, high-pressure pipe that delivers fuel to the injection nozzle, injection nozzle that injects fuel into the cylinder, feed pump that sucks fuel from the fuel tank, and fuel filter make up the diesel fuel injection system that filtrates the fuel.

6. Petrol engine works on which cycle?

Petrol engine works on Otto cycle. The petrol engines are also known as gasoline engines.

7. In a diesel engine fuel is ignited by ………..?

Heat produced by compressing the air that is used for combustion

8. What is the main difference between diesel and petrol engines?

The main difference lies in their ignition methods. Petrol engines use spark ignition (Otto cycle), where a spark plug ignites the fuel-air mixture. Diesel engines use compression ignition (Diesel cycle), where high compression of air causes the fuel to ignite when injected.

9. What is the Otto cycle, and how does it relate to petrol engines?

The Otto cycle is the thermodynamic cycle used in spark-ignition internal combustion engines, typically petrol engines. It consists of four strokes: intake, compression, power, and exhaust. The cycle is named after Nikolaus Otto, who invented the first practical four-stroke engine.

10. How does the Diesel cycle differ from the Otto cycle?

The Diesel cycle, used in compression-ignition engines, differs from the Otto cycle in that it has a constant-pressure heat addition phase instead of a constant-volume heat addition. This results from injecting fuel gradually into the compressed air, rather than having a pre-mixed fuel-air charge.

11. Why do diesel engines have higher compression ratios than petrol engines?

Diesel engines have higher compression ratios to generate enough heat to ignite the fuel without a spark. The higher compression ratio also contributes to their greater fuel efficiency and torque output compared to petrol engines.

12. Why are diesel engines generally more fuel-efficient than petrol engines?

Diesel engines are more fuel-efficient due to their higher compression ratios, which allow for more complete fuel combustion. They also operate at lower engine speeds and have a more energy-dense fuel, contributing to better overall efficiency.

13. How does the fuel injection process differ between diesel and petrol engines?

In petrol engines, fuel is typically injected into the intake manifold or directly into the cylinder during the intake stroke. In diesel engines, fuel is injected directly into the cylinder at high pressure near the end of the compression stroke, just before ignition.

14. What is the significance of the air-fuel ratio in petrol and diesel engines?

The air-fuel ratio is crucial for efficient combustion. Petrol engines typically operate at or near the stoichiometric ratio (about 14.7:1), while diesel engines run with excess air (lean mixture). This difference affects emissions, fuel efficiency, and power output.

15. How do the emissions from diesel and petrol engines compare?

Diesel engines generally produce lower CO2 emissions due to better fuel efficiency, but they tend to emit more particulate matter (PM) and nitrogen oxides (NOx). Petrol engines typically produce more CO2 but less PM and NOx.

16. Why do diesel engines produce more torque than petrol engines?

Diesel engines produce more torque due to their higher compression ratios, longer piston stroke, and the way fuel is injected and combusted. The slower-burning, high-pressure combustion in diesel engines results in more force being applied to the crankshaft over a longer duration.

17. Why are glow plugs used in diesel engines but not in petrol engines?

Glow plugs are used in diesel engines to preheat the combustion chamber, aiding cold starts. They're not needed in petrol engines because the spark plug provides the ignition source, regardless of engine temperature.

18. What is the difference in fuel composition between diesel and petrol?

Diesel fuel consists of heavier hydrocarbons (typically C10 to C15) and has a higher energy density than petrol. Petrol contains lighter hydrocarbons (typically C4 to C12) and additives to improve performance and reduce emissions.

19. What is the cetane number in diesel fuel, and why is it important?

The cetane number indicates how quickly diesel fuel ignites under compression. A higher cetane number means faster ignition, which can lead to smoother engine operation, easier cold starting, and potentially lower emissions.

20. How does the octane rating of petrol relate to engine performance?

The octane rating indicates a fuel's resistance to "knocking" or pre-ignition in petrol engines. Higher octane fuels allow for higher compression ratios and more advanced ignition timing, potentially improving engine performance and efficiency.

21. What is the difference in engine speed (RPM) between typical diesel and petrol engines?

Diesel engines typically operate at lower RPMs (usually below 5000 RPM) compared to petrol engines, which can often reach higher RPMs (up to 7000 RPM or more in some cases). This is due to the differences in combustion processes and engine design.

22. What is the difference in fuel atomization between diesel and petrol engines?

In petrol engines, fuel is atomized in the intake system or in the cylinder during the intake stroke. In diesel engines, fuel is atomized at very high pressure directly into the combustion chamber near the end of the compression stroke, requiring more sophisticated injection systems.

23. What is the purpose of a turbocharger in diesel and petrol engines?

Turbochargers increase engine power and efficiency by forcing more air into the combustion chamber. This allows for more fuel to be burned, increasing power output. They are common in diesel engines and increasingly used in petrol engines for improved performance and fuel economy.

24. How does the thermal efficiency of diesel engines compare to that of petrol engines?

Diesel engines generally have higher thermal efficiency than petrol engines due to their higher compression ratios and the nature of the Diesel cycle. This means they convert a larger proportion of the fuel's chemical energy into mechanical work.

25. How does the cost of manufacturing diesel engines compare to petrol engines?

Diesel engines are generally more expensive to manufacture due to their need for stronger components to withstand higher compression ratios and pressures. They also require more sophisticated fuel injection systems and often include turbochargers.

26. How do the lubrication requirements differ between diesel and petrol engines?

Diesel engines typically require oils with higher viscosity and better soot-handling capabilities due to their higher operating pressures and temperatures. Petrol engines generally use lighter oils as they operate at lower pressures and produce less soot.

27. Why are diesel engines more commonly used in heavy-duty applications?

Diesel engines are preferred in heavy-duty applications due to their higher torque output, better fuel efficiency, and greater durability. These characteristics make them well-suited for tasks requiring sustained power, such as in trucks, construction equipment, and ships.

28. How does the power stroke differ between diesel and petrol engines?

In petrol engines, the power stroke begins with the spark ignition of the pre-mixed fuel-air charge. In diesel engines, it starts with the injection and spontaneous ignition of the fuel in the compressed air, resulting in a more gradual pressure rise.

29. What is the impact of fuel quality on diesel and petrol engines?

Both engine types are affected by fuel quality, but diesel engines are generally more sensitive. Poor quality diesel fuel can lead to injector clogging and increased wear, while low-quality petrol can cause knocking and reduced performance.

30. How do cold starting characteristics differ between diesel and petrol engines?

Diesel engines typically have more difficulty starting in cold conditions due to the need for higher compression temperatures to achieve ignition. Petrol engines, with their spark ignition, are generally easier to start in cold weather.

31. What is the difference in noise levels between diesel and petrol engines?

Diesel engines are typically louder than petrol engines due to their higher compression ratios and the nature of compression ignition. However, modern diesel engines have significantly reduced noise levels through improved design and sound insulation.

32. How does the use of alternative fuels differ between diesel and petrol engines?

Petrol engines can more easily adapt to alternative fuels like ethanol or natural gas. Diesel engines can use biodiesel, but may require modifications for other alternatives. Both types are seeing development in hybrid and electric variants.

33. What is the difference in valve timing between diesel and petrol engines?

Valve timing in diesel engines is generally simpler than in petrol engines. Diesel engines often don't require variable valve timing systems, which are common in modern petrol engines to optimize performance across different operating conditions.

34. How do the cooling requirements differ between diesel and petrol engines?

Diesel engines typically generate more heat due to higher compression ratios and often require more robust cooling systems. Petrol engines generally have lower cooling demands but may need more precise temperature control for optimal performance.

35. What is the significance of the compression ratio in determining engine efficiency?

Higher compression ratios generally lead to greater thermal efficiency as they allow for more energy to be extracted from the fuel. This is why diesel engines, with their higher compression ratios, are typically more efficient than petrol engines.

36. How does fuel injection pressure compare between diesel and petrol engines?

Diesel engines use much higher fuel injection pressures (often exceeding 2000 bar in modern systems) compared to petrol engines (typically 2-200 bar). This high pressure is necessary for proper fuel atomization and mixing in diesel engines.

37. What is the difference in air intake systems between diesel and petrol engines?

Diesel engines typically use simpler air intake systems as they only compress air during the intake stroke. Petrol engines often have more complex intake systems to control the air-fuel mixture, including throttle bodies and sometimes variable intake geometry.

38. How do the pistons differ in design between diesel and petrol engines?

Diesel engine pistons are generally heavier and more robust to withstand higher compression pressures. They often have a different crown shape to create the necessary turbulence for fuel mixing. Petrol engine pistons are typically lighter with simpler crown designs.

39. What is the impact of altitude on the performance of diesel vs petrol engines?

Petrol engines tend to lose power more quickly at high altitudes due to reduced air density. Diesel engines, especially turbocharged ones, are less affected by altitude changes because they can compensate more easily for lower air density.

40. How does the use of forced induction (turbocharging/supercharging) differ between diesel and petrol engines?

Forced induction is more commonly used in diesel engines to increase power density and efficiency. In petrol engines, it's often used for performance enhancement. Diesel engines can handle higher boost pressures due to their stronger construction.

41. What is the difference in exhaust gas recirculation (EGR) systems between diesel and petrol engines?

Both engine types use EGR to reduce NOx emissions, but diesel engines typically require higher EGR rates. The implementation can differ, with diesel EGR systems often needing additional cooling due to higher exhaust temperatures.

42. How does the combustion chamber design differ between diesel and petrol engines?

Diesel engine combustion chambers are designed to create turbulence and promote fuel-air mixing. They often have a bowl-in-piston design. Petrol engine combustion chambers are simpler, focusing on flame propagation from the spark plug.

43. What is the difference in fuel pump designs between diesel and petrol engines?

Diesel engines use high-pressure fuel pumps capable of generating pressures up to 2500 bar or more for common rail systems. Petrol engines use lower pressure pumps, typically electric in-tank pumps for fuel injection systems.

44. How do the maintenance requirements differ between diesel and petrol engines?

Diesel engines often require more frequent oil changes and fuel filter replacements due to higher oil contamination and stricter fuel cleanliness requirements. Petrol engines typically have longer service intervals but may need more frequent spark plug replacements.

45. What is the difference in engine longevity between diesel and petrol engines?

Diesel engines generally have a longer lifespan due to their more robust construction and lower operating speeds. However, this gap has narrowed with improvements in petrol engine technology and the increased complexity of modern diesel engines.

46. How does the use of multi-point fuel injection in petrol engines compare to direct injection in diesel engines?

Multi-point injection in petrol engines injects fuel into the intake ports, while direct injection in diesel engines injects directly into the cylinder. Direct injection allows for more precise fuel control and is becoming more common in modern petrol engines as well.

47. What is the significance of the expansion ratio in the Otto and Diesel cycles?

The expansion ratio in both cycles affects the amount of work extracted from the combustion gases. In the Otto cycle, it's equal to the compression ratio, while in the Diesel cycle, it can be higher, contributing to the cycle's efficiency.

48. How does the use of variable geometry turbochargers differ between diesel and petrol engines?

Variable geometry turbochargers are more common in diesel engines, where they help provide good low-end torque and reduce turbo lag. They're less common in petrol engines due to higher exhaust temperatures, though this is changing with advances in materials and design.

49. What is the difference in knock resistance between diesel and petrol engines?

Petrol engines are more susceptible to knocking (pre-ignition) and require fuel with appropriate octane ratings to prevent it. Diesel engines are less prone to knocking due to their compression-ignition nature, but can experience "diesel knock" under certain conditions.

50. How does the use of intercoolers differ between turbocharged diesel and petrol engines?

Both turbocharged diesel and petrol engines use intercoolers to cool the compressed intake air. However, diesel engines often require larger, more efficient intercoolers due to higher boost pressures and the greater importance of air density for combustion.

51. What is the difference in throttle control between diesel and petrol engines?

Petrol engines use a throttle valve to control air intake and thus power output. Diesel engines typically don't use a throttle valve; instead, power is controlled by varying the amount of fuel injected into the cylinder.

52. How does the use of catalytic converters differ between diesel and petrol engines?

Petrol engines use three-way catalytic converters that can reduce CO, HC, and NOx emissions simultaneously. Diesel engines typically use oxidation catalysts and separate systems (like SCR) for NOx reduction due to their lean-burn operation.

53. What is the difference in cold-start emissions between diesel and petrol engines?

Petrol engines typically produce higher cold-start emissions of hydrocarbons due to fuel condensation in cold cylinders. Diesel engines have lower cold-start HC emissions but may produce more visible smoke (particulates) until they reach operating temperature.

54. How does the use of engine braking differ between diesel and petrol engines?

Diesel engines are more effective at engine braking due to their higher compression ratios and the lack of a throttle plate. This makes them particularly useful in heavy vehicles. Petrol engines can also use engine braking, but it's generally less effective.

55. What is the difference in fuel vaporization between diesel and petrol engines?

Petrol is more volatile and vaporizes more easily, which is necessary for mixing with air before combustion. Diesel fuel is less volatile and relies on high-pressure injection and in-cylinder conditions for proper atomization and mixing.

56. How does the use of synthetic oils differ between diesel and petrol engines?

Both engine types can benefit from synthetic oils, but the requirements differ. Diesel engines often require oils with higher viscosity and better soot-handling capabilities. Synthetic oils in petrol engines focus more on reducing friction and maintaining cleanliness.

57. What is the difference in engine weight and size between comparable diesel and petrol engines?

Diesel engines are typically heavier and larger than petrol engines of similar power output. This is due to the stronger construction needed to withstand higher compression ratios and pressures, as well as additional components like high-pressure fuel systems.