Fleet 101: Understanding Direct vs. Port Injection in Engines

Truck engines have changed a bit since the first automobiles hit the road, but the basic premise for gasoline engines is the same: you need fuel, oxygen, and a spark to make it work. At the fundamental level, fuel is delivered by injecting it via a calculated burst of a fine spray into the combustion chamber of an engine.

Initially, the way to mix the fuel with air was through a carburetor, patented in 1872. With the updated needs of catalytic converters, carburetors were no longer efficient. Today, we use fuel injection, and there are two main ways to make this mixture happen in an internal combustion engine – direct injection or port injection.

But, is port injection better than direct injection? And just what are the advantages of direct injection over port injection?  

Fleet 101 with Work Truck: Direct Injnection Engine vs. Port Fuel Injection Engine

First, we start with the basics: What is direct injection?

Direct injection engines have been around for quite some time, used in fighter aircraft in World War II. It is the fuel delivery method where fuel is injected directly into the combustion chamber. 

“With a direct injection engine, the fuel is atomized by the extreme pressure used to inject it – up to 2,200 pounds per square inch (psi) on the all-new 6.6L V-8 gas engines offered on the 2020 Chevrolet Silverado HD,” explained Mike Kociba, assistant chief engineer of Small Block engines for General Motors.

Port fuel injection has been around since the 1980s and means that fuel is delivered to the engine directly into the intake manifold or cylinder head. Fuel is sprayed on the valve, which then uses the heat from the valve to further atomize the fuel. 

“Both systems focus on atomizing the fuel for more efficient combustion of the fuel. The difference is how they atomize the fuel: direct injection uses very high pressure and is sprayed directly into the spark plug area to ignite. Port fuel injection uses heat from the valves to atomize the fuel before being ingested into the cylinder when the valve opens,” Kociba said.

The most significant advantage of direct injection is it injects a cooler air/gas mixture into the cylinder. That reduction in heat has two benefits.

“First, it enables a higher compression ratio which offers higher performance and efficiency. Second, it enables better engine performance at cold starts, which is especially important in cold, northern climates,” Kociba noted.

The biggest advantage of port fuel injection is it naturally cleans the valves with every spray of fuel.

“For direct injection engines, we have engineered sophisticated systems to prevent valve build-up – including a positive crankcase ventilation system that helps prevent oil from depositing on the valves,” he added.

One disadvantage of port injection is that fuel can puddle and be absorbed by surrounding areas, making it harder to control.

General Motors has been using direct injection engines for more than a decade.

When it comes to whether direct injection is more efficient, they believe they know the answer: 

“GM went with direct injection engines because it delivers impressive levels of performance, efficiency, and durability. We have applied all the expertise we have developed through generations of direct-injection designs to the all-new 6.6L gas engines on the Silverado HD,” Kociba concluded.

Editor's Notes: Image updated Dec. 2, 2019; content updaqted Jan. 30, 2023

Originally posted on Work Truck Online