How Gasoline Direct Injection Engines Work - over 70 MPG without a hybrid drive

by | Oct 22, 2010 07:00 AM ET

All major car manufacturers appear to be switching over to a new gasoline engine technology called gasoline direct injection. And they are making some pretty amazing claims. For example, Mazda is claiming 70 miles per gallon from its new engine/transmission/body combo called SKYACTIV:

Next-Gen Mazda2 Will Return 70 MPG, Without An Electric Motor

It will come powered by a SKYACTIV-G engine, Mazda's next-generation direct injection gasoline mill that achieves significantly improved fuel efficiency thanks to a high compression ratio of 14.0:1 (the world's highest for a production gasoline engine). In addition to the improved fuel economy, Mazda also claims that the higher compression ratio enables more torque, especially at lower to mid-range engine speeds, which should make the car a whole lot more fun to drive around the town.

On YouTube it is possible to find videos from many of the major manufacturers on their new GDI engines.


Holden SIDI:

Ford EcoBoost:


Nissan GDI:

Hyundai Theta engine:

There are several common threads that you see in these videos:

  1. The engines use injectors that can spray fuel directly into the cylinder during the compression stroke, along with an extremely high pressure fuel pump (2,000 PSI). Before GDI, it was far more common to use port fuel injection, where the injector sprayed fuel at low pressure into the intake manifold.
  2. The direct injection process allows the fuel to evaporate in the cylinder and cool the air/fuel mixture. That helps avoid premature ignition, so...
  3. These engines can increase the compression ratio. The Mazda engine goes to a 14:1 ratio, which has never been seen before in a production gasoline engine. The normal high is 12:1 or so, and that would require premium fuel.
  4. Many of the engines are using multiple injector sprays per stroke. One spray occurs as the air starts flowing in on the intake stroke. The second occurs right before the spark plug fires. This creates a stratified charge of fuel for a better burn pattern.

The high compression ratio allows more energy to be extracted. Think of it this way: when gasoline burns in a higher-compression environment, it gets to expand further, meaning that more work can be extracted from the expansion. The stratified charge allows less fuel to be injected while still getting a good ignition/burn pattern. It allows a much leaner mixture in the cylinder.

This paper, published in 2005, discusses the advatage of stratified charge:

Port fuel injected (PFI) engines are the most commonly used
spark ignition (SI) engine in current vehicles. In certain
markets, a very small number of direct-injected spark ignition
(DISI) engines have been introduced. Both use gasoline fuel. In
PFI engines, fuel is injected into the intake port near the closed
intake valve, producing a well mixed fuel–air charge in the
combustion chamber. This is the most commonly used engine
type in current vehicles. These engines are typically operated
with a stoichiometric fuel–air ratio, which is the ratio that
permits complete conversion of the fuel and oxygen in the
intake charge to form CO2 and H2O. As a result of the premixed
combustion, it produces very low particulate emissions.
The levels of other emissions directly leaving the engine are
relatively high, and compliance with regulated emission
standards relies on the effectiveness of the three-way catalyst,
which reduces emissions by 95–99% as discussed in more detail
In DISI engines, the fuel is injected directly into the
combustion chamber. At higher load, the fuel is injected
during the intake stroke to form a nearly homogeneous fuel–
air mixture at the time of ignition. At lower load, the injection
timing can be delayed until the compression stroke to produce
a ‘‘stratified'' fuel mixture. This mixture is ideally uniform,
premixed, and stoichiometric near the center, and devoid of
fuel near the cylinder walls. This spatial localization translates
into a faster burn and allows the engine to be run more fuellean
overall than PFI engines, providing improved fuel
economy and better performance during transient acceleration/
deceleration. In practice, however, it is difficult to realize
this idealized mixing, and fuel-rich and lean regions result,
leading to reduced benefits. Additionally, because this engine
injects fuel droplets directly into the combustion chamber,
particulate emissions are increased substantially relative to PFI
engines.5 Like PFI engines, DISI engines rely on catalytic
devices to significantly reduce engine-out concentrations of
regulated emissions.

Apparently, some of the kinks with non-idealized mixing and particulates have been figured out since 2005.

This page talks about the advatages of higher compression ratios:

The higher the compression ratio, the more closely packed the molecules of fuel and air are when the mixture is ignited by the sparkplug, this causes a more powerful explosion by making a more violent reaction which produces more power. Higher compression makes the expansion ratio of the exploding hot gas greater which means that more energy is impinged on the piston top, pushing it down harder, making more power. Increasing the compression ratio improves the thermal efficiency of an engine and this is the primary reason why higher compression increases power. Improving thermal efficiency improves fuel economy from getting more power from the same amount of fuel and a reduction of combustion chamber surface area to volume. This means less wasted combustion heat and more expansion being used to drive the piston down.

This page talks about the new piezoelectric fuel injectors being used for gasoline direct injection, plus it talks about an unexpected advantage of GDI - engine starting without needing to use the starter motor:

Convinced that a stop-start function will provide real fuel savings, but cognizant of automakers' unwillingness to pay to integrate a separate electric drive starting system, Bosch created “Directstart” for GDI engines. It starts a combustion engine by igniting a fuel-air charge without engaging the starter motor. “Fuel efficiency is improved by up to 5%,” says Leonhard, “and the starting is cleaner, quieter, and faster, while lessening the effort that would have to be exerted by the starter motor in extremely low ambient temperature conditions, or on motors with a large number of cylinders.” OEMs have the option of downsizing their current starter motors, or using a more frequent stop/start schedule with their current designs.

With these new GDI engines, it may be possible to create gas engines that are nearly as efficient as fuel cells.

- How Fuel Cells Work

- How Engines Work

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