By the thermodynamics of the internal combustion engine (Carnot cycle), the higher the compression ratio, the more mechanical work is derived from the gasoline burned.

So why is it not common practice to mechanically inject gasoline into the cylinder after the air is compressed, similar to diesel engines? IE no spark, but rather the high temperature created during compression (or a glow plug) causes the air to spontaneously combust with the fuel.

There is no issue with knocking, because the the fuel is introduced after compression is complete (IE, at or after top dead center.. or just slightly before).

What else is going on that blocks this approach?

Is there an issue with rate of combustion? IE, gasoline doesn't burn fast enough, so combustion ends up incomplete if fuel is injected near top-dead-center?

Or.. is there an issue with control over the amount of fuel? IE, to be efficient, you need roughly 14.7 to 1 air to fuel.. so have to adjust the fuel mechanically injected according to the air density in the cylinder at the position that the intake valve is closed. Is there no good mechanism to have this precise control? Like, too expensive or too bulky or too unreliable or something?

Or.. some other factor?

Or.. am I missing something? Is the whole premise wrong, that direct mechanical injection of fuel around top dead center would actually not be more fuel efficient?

  • $\begingroup$ Depending on the speed of the engine your "inject slightly before TDC" needs to be more than slightly - check out timing advance... And also consider the "fixed" combustion time for a fuel charge. $\endgroup$
    – Solar Mike
    Mar 15, 2023 at 7:07
  • 1
    $\begingroup$ I'm tempted to say "if you're using a piston-based internal combustion engine, you've already decided that fuel efficiency is not a priority". $\endgroup$ Mar 15, 2023 at 13:07

3 Answers 3


First of all, a frame challenge. Saying that it isn't done is just wrong. Fuel economy in gasoline engines implies a big, sophisticated turbocharger installation. It works best with maximum volume, so you want a large amount of excess air in the cylinder. Gasoline isn't terribly fond of lean mixtures. The classical solution is a precombustion chamber that is rich, followed by stratified combustion. You can use a separate injector for the prechamber, or just get fancy with single injectors.

Compression ratios can be nearly anything you want them to be. Emissions laws and pressure on gasoline formulations from refineries push the compression ratio down to maximize fuel economy, meaning the cost of fuel per mile. Consider a 1.6 liter V6 making about 35 psia from the charger, running 18:1 mechanical compression. Induction valve timing limits actual compression to about 15:1. Single (really fancy) direct injection - 850 hp from about 10,000-13,000 rpm on basically pump gas. This works, but F1 isn't terribly concerned about the cost of the fuel or the maintenance requirements due to an extreme lean-burn fuel map. The cost of these engines is believed to be more than 10 million USD each. If you can figure out how to reduce that by a factor of 10,000 then you might be on to something.

High compression engines tend to be heavier than low compression engines making the same power. This is a major factor in mobile applications, where the weigh increase cascades through the entire vehicle structure. Naturally aspirated motocross single cylinder bikes (unrestricted displacement) used to run about 7:1 compression due primarily to weigh considerations*. So the design space is rather flat from a compression ratio standpoint. Somewhere between 5:1 and 30:1 is doable and practical under some circumstances. Any regulatory matters will tend to lock the compression ratio against a wall defined by how the cost of compliance changes with compression ratio.

Some other random factoids - Mercedes built a 4.4:1 engine in 1909. Ford's flathead V8 started production at 5.5:1. Getting a high pressure engine cranked is a serious problem - the bearing loads are ridiculous when the first cylinder lights off. Lubrication systems and cranking systems and bearing arrangements all have to be more sophisticated as peak cylinder pressures rise. Useable rpm range and "driveability" are reduced with higher compression ratios. Thermal losses in the iron increase with compression ratio, eliminating some, or all, of the theoretical efficency benefits posited by the ideal cycle thermodynamics. But this is a desirable feature of small undersquare engines that need to limit upper cylinder temps.

*A peculiar issue with these engines is that lots of torque ripple is highly desirable, which also favors big slow long-stroke singles.


Gasoline (Petrol) and Diesel are different fuels and perform best in different engines.

Gasoline is best lit by a spark. Its autoignition is too imprecise to be used to start combustion. It would be too difficult to time the ignition, and the resulting combustion would be dirty. This would necessitate a long stroke as well to allow complete combustion, which limits RPM. The last factor is that since an engine built for such high compression & autoignition is so hefty, you further limit RPM, which is necessary to gain higher power. Power is what consumers want in an engine, which is why people prefer to drive gasoline engines. Overbuilt long stroke engines are going the wrong way from what makes a gasoline engine good.

Autoignition is knock. Diesel engines knock like crazy. After TDC autoignition isn't an issue in Spark Ignition engines because we have already lit the charge. Note that knock occurs also in the chamber on the other side of the flame front as combustion pressurizes the cylinder. One way to fight knock is higher engine speeds, but see above if we choose to design a low speed gasoline engine. Part of why diesel engines are built so sturdily is to account for the stress of constant autoignition.

As to running very lean, this is a problem with flame propagation speed (resulting again in a longer stroke). But the main problem is a catalytic converter doesn't work well in lean conditions, and you get increased NOx emissions. Counter technology used in diesel engines is fairly intrusive and not desirable.

Basically, injecting fuel into high pressure to ignite it is a diesel thing, so leave it to diesel engines running diesel fuel, which are made for it, and which have their own use cases.


You are correct, and I apologize for the confusion. The auto-ignition temperature of gasoline is indeed higher than that of diesel [1]. I appreciate the opportunity to clarify and provide a revised response.

Flash point: Gasoline has a lower flash point than diesel, which means it evaporates more quickly and forms flammable mixtures with air more easily. This property is critical in spark-ignition gasoline engines, which rely on the formation of a homogeneous air-fuel mixture before combustion. In compression-ignition diesel engines, fuel is injected directly into the cylinder, and the fuel-air mixture forms in the combustion chamber [2].

Octane rating: Gasoline has a higher octane rating than diesel fuel. High octane fuels resist knock or auto-ignition better than low octane fuels. Therefore, gasoline engines usually employ spark ignition to control the combustion process [3].

Combustion rate: Gasoline has a faster combustion rate compared to diesel. In a compression ignition engine, the fuel needs to be injected and burn almost instantaneously for efficient combustion. Gasoline's faster combustion rate makes it challenging to achieve complete combustion in a compression ignition engine [1].

Emissions: Diesel engines can produce high levels of particulate matter and nitrogen oxides (NOx) emissions. Compression ignition gasoline engines can also produce high NOx emissions due to the high combustion temperatures [4]. While GDI engines can offer better fuel efficiency, they can also produce more particulate matter emissions compared to conventional gasoline engines. Emission control technologies and regulations have to be considered when developing these engines.

Control and complexity: Achieving the right air-fuel ratio in a gasoline compression ignition engine can be challenging. Precise control of the injection timing, fuel pressure, and combustion temperature is required, which can increase the complexity and cost of the engine management system [5].

In summary, while gasoline direct injection technology is increasingly used in modern gasoline engines, the compression ignition approach remains more common in diesel engines due to the reasons mentioned above. Researchers continue to explore advanced combustion strategies that could potentially combine the benefits of both gasoline and diesel engines, like Homogeneous Charge Compression Ignition (HCCI) [6].


  • [1] Heywood, J. B. (1988). Internal combustion engine fundamentals. New York: McGraw-Hill.
  • [2] Stone, R. (1999). Introduction to Internal Combustion Engines. Macmillan International Higher Education.
  • [3] Kalghatgi, G. (2018). Developments in internal combustion engines and implications for combustion science and future transport fuels. Proceedings of the Combustion Institute, 37(1), 101-115.
  • [4] Alkidas, A. C. (2007). Combustion advancements in gasoline engines. Energy Conversion and Management, 48(11), 2751-2761.
  • [5] Zhao, H. (2010). HCCI and CAI engines for the automotive industry. Elsevier.
  • [6] Yao, M., Zheng, Z., & Liu, H. (2009). Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Progress in Energy and Combustion Science, 35(5), 398-437.
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    $\begingroup$ gasoline has a higher autoignition temperature than diesel $\endgroup$
    – Tiger Guy
    Mar 16, 2023 at 16:37
  • $\begingroup$ Agreed, the flashpoints are significantly different. I will edit my answer to discuss flash point differences. $\endgroup$ Mar 16, 2023 at 17:00

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