Making a petrol engine work by modulating fuel injection?

In a diesel engine the intake is unrestricted. Pressing the gas pedal injects more fuel. Is it possible to make a 4 stroke petrol engine work like that?

They do. Gasoline spark-ignited (SI) port-injected engines modulate air and fuel intake in response to pressing on the accelerator pedal.

If you were talking about making a gasoline SI engine in which air intake is unrestricted, yes, some gasoline direct-injection engines do this, but it’s a pretty recent innovation.

I forgot about homogeneous charge compression ignition (HCCI) engines. No throttle plate, like a diesel. UNlike a diesel, they inject fuel into the intake port, and the intake event draws a homogeneous fuel-air mixture into the combustion chamber. Ignition is by compression (HCCI engines have high compression ratios, just like diesel engines). As it turns out, this is extremely difficult to control: the intake mixture’s air/fuel ratio has to be just right, and so does its temperature, in order for it to spontaneously combust at just the right moment near the end of the compression stroke. With the use of EGR, the outcome of one combustion event can affect future combustion events (combustion chamber surface temp matters too), so it’s possible for the whole process to run awry very quickly without constant adjustments by the ECU. Typically the ECU examines the pressure trace from each combustion event and fine-tunes the boost, EGR, intake temperature (via powerful heaters), and fuel injection quantity to ensure that the next combustion event (phasing, duration, total heat release, etc.) stays within prescribed parameters.

HCCI is the holy grail, though: the lean burn (lots of excess air) and unthrottled intake mean there is the potential for efficiency values normally associated with diesel engines, and the homogeneous mixture keeps PM/NOx emissions on par with conventional gasoline engines.


Some port injected gasoline engines control engine speed by modulating injector pulses under some conditions. The ECU will command the throttle wide open and control engine speed with injector pulse, ignition timing, and valve timing.

What ? why ?

Did HCCI do beat the restriction on efficiency imposed by the Carnot cycle theory ? No.

Petrol engines always burn their valves when they run lean. The spark plugs don’t like lean either.

So the butterfly valve stops the intake of air, so that a tiny bit of fuel can keep the cylinders from being lean.

The butterfly valve throttle causes no significant problem with efficiency, so no need to do away with it.
Diesel’s get good efficiency by preventing the driver from taking it to 6000 rpm ( well its not the lack of spark plugs… spark plugs do not cause a fuel waste ! !)

No, but it comes a lot closer to it than conventional gasoline SI engines.

All other things being equal, a lean mixture has a lower flame temperature than a stoichiometric mixture. The problem with a lean mixture in a spark-ignited gasoline engine is that the flame speed is a lot lower (than for a stoichiometric mixture), so if the spark timing isn’t advanced to compensate for that slow burn, then the bulk of the combustion happens later in the expansion stroke. Then there’s less opportunity to extract heat from the mixture (as mechanical work to the crankshaft), so you actually do end up with hotter exhaust temperatures that can fry valves and overheat engines. If you can adjust the spark timing to compensate for the slower burn, a lean-burn spark-ignited gasoline engine will be more efficient than an engine operating on a stoichiometric mixture - because the lower peak temperatures mean less heat rejection to the engine block/head.

If you crack open a textbook on IC engines, you will see that you are flat-out wrong on this.

Three key reasons diesels are more efficient than spark-ignited gasoline engines:

1. no throttle plate. Yes, this matters; sucking air through a restriction is a big reason for why SI gasoline engines have lower efficiency than diesels.

2. lean combustion. Because diesel engines have lots of excess air compared to the amount of fuel that gets injected, combustion chamber temperatures are lower, so you lose less heat to the combustion chamber surfaces, leaving more heat available to do mechanical work during the power stroke. Exhaust gas recirculation is employed to reduce emissions in SI-gas engines, but it also has a similar effect: the added mass of the EGR absorbs heat from the combustion event, lowering peak temperatures and making the SI-gas engine more efficient than it otherwise would be.

3. high compression ratio. The issue isn’t really compression ratio so much as it is expansion ratio. The amount that hot combustion gases are allowed to expand during the expansion stroke dictates how much of the available heat energy can be converted to work. Conventional gasoline spark-ignited engines are limited to something like 12:1, but diesels operate at around 19:1.

The real downer with diesel engines is the inhomogenous combustion. You squirt cold fuel into screaming hot air, and you get rich spots that produce soot, and lean spots that produce NOx. We want a homogeneous mixture so as to minimize these pollutants, but how can we use a homogenous mixture and still control power output, while also keeping the things that confer such great efficiency on a diesel engine (no throttle plate, high compression ratio, and lean combustion)?

And that’s why HCCI is so interesting. Because it utilizes compression-ignition, we’re not dependent on a contiguous, propagating flame front, so we can use really lean mixtures that wouldn’t burn in a spark-ignited engine. Because there’s no throttle plate, pumping losses are as low as for a diesel; we control load by modulating the fuel quantity. And since we’re relying on compression ignition, we can go ahead and use that same high compression ratio that we find in diesel engines.