Your Guide to the Differences Between MPFI and Direct Injection Fuel Systems

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Your Guide to the Differences Between MPFI and Direct Injection Fuel Systems

In the old days, vehicle engines used carburettors to provide the oxygen for the fuel air mixture. This mixture provides the combustive power that drives the vehicle down the road. These combustive explosions, on the order of hundreds of times per minute, impart this explosive energy to the crankshaft, which in turn drives the wheels of the vehicle, setting it in motion after applying the gas pedal.

Recent motor vehicles use fuel injection as opposed to a carburettor. Due to the improved efficiency and fuel mileage of a fuel injection system, as opposed to that of a carburettor, fuel injectors have become the new way to deliver the fuel for the fuel and air mixture used for engine combustion. Technology in this area has increased tremendously over the years. What started out as a single-point injection system grew to utilize a multi-point injection system and then evolved to the current preferred injection method, direct injection. And while a Multi-Point Fuel Injection (MPFI) system and a Direct Fuel Injection (DFI) system exhibit many similarities, there are also some key differences.

Common Types of Fuel Injection Systems

A multitude of fuel injection system types exist, though most fall into three distinct categories: single-point injection, multi-point injection, and direct injection. The table below provides details on these three common types of fuel injection systems.

Fuel Injection System Types

Description

Throttle Body

Single-point injection system; costs less; simplest fuel injection system available

Multi Point Fuel Injection

Injects fuel into each cylinder's intake valve; can be in sequence, simultaneously, or in groups

Direct Fuel Injection

Directly injected into the cylinder without mixing first; costs more; very clean system since intake is dry

While there are more than just the above three fuel injection systems available, the three detailed systems are used with motor vehicles, whereas the others find more common use in aircraft engines and those of high-end sports cars. Some fuel injection systems no longer see common use, such as the central port injection system employed by General Motors from 1992 to 1996.

How Fuel Injection Works

Early fuel injection systems used only one point of injection to add the fuel to the fuel and air mixture. This adding of fuel was accomplished on the intake stroke of the four-stroke engine process. These fuel injection systems were less costly than current systems, but they also used the same throttle body of the carburettor, which was an inefficient way to deliver fuel to the engine.

With the introduction of the MPFI system, fuel was delivered to outside the cylinder, just upstream of the intake valve for that cylinder. The actual fuel injection process of the multi-point system broke down into three types of injection methods, sequential, batched, and simultaneous.

Sequential is timed to coincide with the intake stroke of each cylinder. So, as each cylinder fires and the next one prepares to follow in succession, the fuel is injected. Batched, which means the fuel injection system delivers the fuel to the cylinders in groups. Simultaneous means the fuel arrives at each cylinder at the same time.

Direct injection, on the other hand, delivers the fuel directly to the combustion chamber. Whereas the single-and multi-point injection systems mix the fuel and air before placement in the combustion chamber, direct injection places the fuel directly into the chamber along with the air for the mixture at the same time.

MPFI and DFI System Components

An Electronic Fuel Injection (EFI) system has several inherent parts that remain the same across the different injector system types. The sections below detail the various components found in a modern EFI system, including injector, fuel pump, engine control unit, and sensors.

Fuel Injectors

The fuel injector sprays the appropriate amount of fuel as directed by the vehicle's on-board computer. The computer determines this according to the driving conditions at the time applied to a map, or program within its systems that details how much fuel the fuel system delivers at any particular time and condition. The fuel injector sprays the fuel at its delivery point in accordance with these computer instructions.

Fuel Pump

The fuel pump sends the fuel from the gas tank through the fuel lines and to the throttle body or fuel rail for delivery to the combustion chamber via the fuel injectors. This item sometimes becomes clogged and must be replaced. It is located either within the fuel tank or at some point outside of the tank in between the fuel tank and the engine. Fuel pumps in the fuel tank require the services of an automotive specialist since the fuel tank needs removal.

Engine Control Unit

The central on-board computer that helps determine the amount of fuel needed according to the driving conditions. The engine control unit,, located within the engine compartment, follows a map, or program that gives the criteria for how much fuel is delivered in various situations.

Sensors

Sensors located within the vehicle fuelling system help the engine control unit determine when certain functions need to happen. The list below contains some of the more common sensor types.

  •     Crank and Cam Position Sensor
  •     Airflow Sensor
  •     Exhaust Gas Oxygen Sensor

    Differences Between MPFI and DFI Systems

    Sharing some of the same features, MPFI and DFI systems exhibit some key differences from each other. These differences mostly occur in how they deliver fuel to the combustion chamber. The following sections give more information on the differences between MPFI and DFI systems.

    Point of Fuel Injection

    Delivery of the fuel to the combustion chamber represents the main difference between the two fuel system types. The MPFI system delivers the fuel to a point close to the intake valve. The fuel and air mix before entering the combustion chamber during the intake stroke. A DFI system delivers the fuel to within the combustion chamber, combining the fuel and air as the air is drawn into the chamber on the intake stroke.

    The direct injection method improves the overall fuel efficiency of the engine over the multi-point fuel injection system. A direct injection system also allows for better fuel control. This in turn lowers the emission levels put out by the vehicle.

    Sequence of Injection

    As mentioned earlier, an MPFI system can operate either using a sequential, batched, or simultaneous injection delivery system. Modern electronic fuel injection systems use a sequential MPFI. Direct injection systems have gained ground on their multi-point predecessors, being placed in newer gasoline engines. Due to the high fuel pressures involved, DFI systems deliver their fuel quickly, as a common fuel rail feeds each cylinder at the same time.

    Fuel Pressure

    Fuel pressure in the DFI system varies greatly from its MPFI system predecessor. The typical fuel pressure produced in an MPFI system runs around 50 to 60 PSI. A DFI system, on the other hand, can produce fuel pressures from around 1000 to as high as 26,000 PSI, which is a big difference and, if care is not taken while servicing a DFI engine, a dangerous amount of pressure.

    Compression Ratio

    When it comes to compression, the higher the compression produced, the more horsepower an engine can put forward as power to run the vehicle. When dealing with the difference in horsepower produced by a DFI engine compared to an MPFI, there is no mistaking the better performance of the DFI system. In comparison, a DFI system produces roughly four to five per cent more horsepower than an MPFI system.

    Cost

    When it comes down to cost, the MPFI system beats a DFI system hands down. A product of the higher heat and stresses placed upon a DFI system as the delivery mechanism is within the combustion chamber, which exposes the DFI system more to the heat and stresses caused when the fuel and air mixture combusts in the chamber.

    Shop for Fuel Injection System Parts on eBay

    To search for items, such as "Ford fuel injection system&" or "direct fuel injection system&", on eBay, go to any eBay page to begin. You can type in any search term that applies to the product needed, including fuel injection system parts. Remember to use terms that specifically describe the product to ensure accurate results. To narrow the results down even more, just click on the appropriate category option. For help on searching the eBay marketplace using keywords, read over eBay's Search Tips page on how to shop for fuel injection systems. Also, eBay shops can provide you with even more product options.

    Remember to read over all product specifications in each listing before making a purchase. And if you have any questions after reading through the product listing, contact the seller directly by clicking on the Ask a Question link on the specific listing page. Also, be sure to assess the seller's feedback ratings from other shoppers. Make sure the seller has a high rating.

    Conclusion

    When shopping for a fuel injection system and fuel injection system parts, shoppers need to keep in mind the type of fuel injection system their vehicle already has installed. Shoppers should purchase parts that match up with the type of fuel injection system they currently own.

    When purchasing vehicles with different fuel injection system types, shoppers need to keep in mind the key differences. An MPFI system costs less than a DFI system, though the overall efficiency of the DFI system is better than the MPFI system. Depending on how often the owner drives the vehicle helps determine which fuel injection system type best fits their driving style. When driving low mileage, an MPFI system proves more cost efficient. Whereas, if driving a vehicle often and far, use a DFI system, which would more than pay for itself in gas savings. Regardless of the fuel injection system type owned, vehicle owners should consider shopping on eBay for fuel injection system part.

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