This is a detailed comparison of three different kinds of fuel injection. Port injection (or PFI), direct fuel injection (GDI or gasoline direct injection) and dual injection, which is a combination of port and direct fuel injection. We will see how each setup works, how they differ from each other and we will of course examine the benefits and drawbacks of each setup, so, let's get started.

So both port and direct injection essentially do the same thing – they inject fuel into the engine to create a combustible air/fuel mixture which when combusted creates combustion pressures which drives the piston downward causing the crankshaft to spin which then ultimately turns the wheels of the car.

As the name implies, port injection, injects fuel into the intake port of the engine, before the intake valve, whereas direct injection injects fuel directly into the combustion chamber, after the intake valves. This means that in the case of port injection you will usually find the fuel injectors somewhere on the intake manifold while in the case of direct injection the injectors will often be either on the valve cover or right underneath the intake manifold bolted to the cylinder head.

Both systems consist of essentially the same parts: a fuel tank, a fuel pump, fuel lines and injectors. However because direct injection injects into the combustion chamber it has to inject against the compression pressures of the engine which means that it must operate at much higher fuel pressures. Direct fuel injection often operates at pressures above 2000 psi or 140 bar. Of course to achieve such high pressures the direct fuel injection system must be more complex. It contains both a low pressure in-tank fuel pump and a high pressure cam-shaft driven fuel pump. The injectors themselves are also much more advanced and expensive because they must be capable of opening and closing extremely quickly against very high fuel pressure. They also need to be capable of surviving the harsh conditions created by combustion since their tips are exposed to it.

Direct injection has the advantage of enabling higher compression ratios because it supplies fresh cool fuel directly into the chamber and it injects it later which means that it spends less time inside the engine which means that it picks up less heat than it would in the case of 'port injection. Less heat means less chances of knock which gives direct injection engines more room to increase the compression ratio which can improve both performance and efficiency.

Another reason direct injection can improve performance and reduce emissions and fuel consumption is its location. Because the injector is inside the chamber it means that the amount of fuel injected is the same as the amount of fuel that gets into its chamber. Port fuel injection injects outside the chamber which means that the amount of fuel released isn't necessarily the amount of fuel that ends up in the chamber. Some of it may stick onto the walls of the intake, some may not make it into the chamber before the valve closes. This reduces injection accuracy and control which can negatively impact emissions and efficiency.

But port injection has the benefit of having the intake valves constantly exposed to fuel, a great solvent. Direct fuel injection never injects onto the back of the valves which over time leads to accumulation of carbon deposits and other gunk from the pcv system. This reduces performance and leads to rough running. Results of prevention methods such as oil catch cans and fuel system additives or valve cleaners are mixed and the need to eventually mechanically clean the valves is inevitable. This of course leads to increased maintenance costs for direct injection engines.

Another potential issue that can occur in direct injection engines is LSPI or low speed pre-ignition. This occurs at low rpm and high load (wide open throttle situations). At low rpm piston speeds are low which leads to poor fuel vaporization in direct injection engines. At the same time the high load means that the ECU instructs the injectors to inject more fuel into the cylinder. The other factor that needs to happen is a particle from the back of the valves or an oil droplet that makes it into the chamber and mixes with the poorly vaporized fuel. The mixture then gets exposed to the very high compression inside gdi engines and boom pre-ignition happens, often leading to catastrophic damage if allowed to persist.

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#d4a #gdi #fuelinjection

00:00 Injection location
02:06 Injecting against compression
04:35 Timing
06:02 Compression ratio
07:48 Knock
10:13 Fuel injected vs Fuel combusted
11:09 Vaporization
13:09 Enough fuel for high rpms?
15:18 Intake valve deposits
17:22 LSPI
19:19 Stack the benefits loose the drawbacks