EFI Conversion - A New Fuel System: -

Date	Discussion
Dec 2022	Carburetted fuel systems operate at relatively low pressure, perhaps 4psi, whilst Electronic Fuel Injection (EFI) systems need to work at a significantly higher pressure to operate properly. Carburettors vaporise fuel, whilst metering it into the engine, by allowing it to be drawn through a fine jet. The jet is mounted in a venturi, where the air being drawn in by the engine accelerates to a high speed and, as it slows down on the far side of the venturi, it creates a low pressure area, or partial vacuum, that "sucks" the fuel through the jet and vaporises it at the same time. Carburettors were developed in the 1870s and, whilst their detail design has evolved significantly, their basic principle of operation hasn't really changed at all. In contrast to carburettors, EFI systems inject metered fuel into the engine at high pressure, instead of having the engine draw the fuel in itself. Injecting fuel through a fine jet also has the useful effect of vaporising it at the same time. EFI fuel systems commonly operate at about 3bar (43.5psi), but some high performance engines may go up to 100psi or more. The fuel injector, itself, is simply an electrically operated "tap" with an outlet jet of a fixed size. The specific volume of fuel delivered, into the engine, with each activation of the injector, is, therefore, highly dependent on the current fuel pressure. (It is also dependent on the length of time the injector is held open, too, but we will get to that later.) However, for an EFI system to operate correctly, the fuel must be maintained at a known pressure and that requires the use of a pressure regulator not unlike the one on the top of your BBQ gas cylinder. So, now we have two new components to consider. We need a high pressure fuel pump capable of delivering a sufficient volume of fuel to a pressure regulator that can then maintain the flow at the required pressure. A fuel pressure regulator can be a separate, standalone device or can be built into another part of the system. In either case, it will usually contain a spring-loaded diaphragm valve that passes fuel straight through until the required pressure is reached and then holds it there by diverting the excess fuel, via a "return line", back into the tank, to be used later. Rhapsody has three fuel tanks and that complicates matters somewhat. The "return line", from the pressure regulator has to be able to send the unused fuel back to the tank from which it is drawn. This ensures that messy and dangerous tank overflows don't occur. At its simplest, a dual set of piping, one for feed and one for return, would be required for each tank. Also, a number of manual or solenoid operated taps would be needed to manage, what would be, a fairly complex system. Solenoid taps are just that - they are ordinary fuel taps that are electrically operated. Each of the three fuel tanks has its own solenoid operated stop tap and each tank also has a low pressure fuel pump to move the fuel to the engine. Only one pair of solenoid taps, with its associated low pressure pump, can be selected at any one time and this is done from a dashboard mounted switch. The solenoid taps are all of the "Normally Closed" type, so that, in the event of an emergency or power failure, all the fuel sources are automatically shut off. This approach simplifies matters considerably but doesn't solve the problem of returning unused fuel from the pressure regulator to the selected tank. The solution is to have yet another, additional, tank, near the engine, that is filled by the low pressure pump from whichever one of the three fuel tanks is currently selected, and which then also receives the returned fuel from the pressure regulator, rather than trying to send that fuel back to the tank from which it was drawn. Such a tank can be quite small since it is always kept filled by the low pressure pumps. In times past, in both boats and other vehicles, gravity was used to feed fuel into the engine. This was done by transferring fuel, by hand generally, from the main fuel tank into a small tank, at the right height, on a nearby bulkhead. This was usually done at the beginning of each day's work and the small auxiliary tank became known as a "Day Tank". Also, vehicles being used in a high performance setting may have separate, small, tanks, to overcome fuel starvation caused by extreme cornering forces. In that application it's called a "Surge Tank". Happily, such tanks are readily available via the tuning suppliers, (on eBay - of course), to the vehicle racing fraternity. So, Rhapsody now has a Day Tank. Its capacity is only 1.25litres and provides fuel, at high pressure, to the regulator (and therefore, the injectors) and then accepts the "returned" fuel to "go around again" without having to return that fuel to the original tank. Now, you may be wondering where the Day Tank gets its high pressure fuel from - it's just a tank, right? Well, no, it is more than that, because it also contains a high pressure fuel pump that is built in. The high pressure fuel pump is a Bosch Type 044. It's a common aftermarket unit that fits directly into the top face of the Day Tank. At full power, it can provide a fuel flow of 200 l/hr at up to 75psi and draw 11 amps. All of which is gross overkill for this application, but it's nice to be operating well away from its performance limits. One concern is that it may turn out to be noisy, but we'll see. High pressure pumps and injectors have very fine apertures and tolerances, which means that the fuel has to be very clean. So, a 40 micron, inline filter unit is included in the design. It consists of an aluminium tube with removable ends, so that the internal filter element can be cleaned and/or replaced. It naturally needs to be compatible with the expected pressure and flow rates. Having assembled all the components for the EFI fuel system, it was surprising how many different sizes and types of connectors are used when, presumably, one type would do. It is epitomised by the old engineer's joke "We love Standards - there are so many to choose from". The solution was to physically take all the component items to Automotive Performance Distributors in Railway Terrace, Mile End, and ask for help. And help they did. They were happy to spend the time to find all the right connectors and adaptors to enable the system to be put together. Thanks, Guys!! In summary, fuel is delivered to the Day Tank from any one of the three main fuel tanks, via their dedicated low pressure pump and solenoid operated tap combination. The Day Tank and high pressure pump then deliver fuel to the injectors, via the pressure regulator, and also accepts the unused fuel from the Return Line, to "go around again".

Date

Discussion

Dec 2022

Carburetted fuel systems operate at relatively low pressure, perhaps 4psi, whilst Electronic Fuel Injection (EFI) systems need to work at a significantly higher pressure to operate properly.

Carburettors vaporise fuel, whilst metering it into the engine, by allowing it to be drawn through a fine jet. The jet is mounted in a venturi, where the air being drawn in by the engine accelerates to a high speed and, as it slows down on the far side of the venturi, it creates a low pressure area, or partial vacuum, that "sucks" the fuel through the jet and vaporises it at the same time. Carburettors were developed in the 1870s and, whilst their detail design has evolved significantly, their basic principle of operation hasn't really changed at all.

In contrast to carburettors, EFI systems inject metered fuel into the engine at high pressure, instead of having the engine draw the fuel in itself. Injecting fuel through a fine jet also has the useful effect of vaporising it at the same time. EFI fuel systems commonly operate at about 3bar (43.5psi), but some high performance engines may go up to 100psi or more.

The fuel injector, itself, is simply an electrically operated "tap" with an outlet jet of a fixed size. The specific volume of fuel delivered, into the engine, with each activation of the injector, is, therefore, highly dependent on the current fuel pressure. (It is also dependent on the length of time the injector is held open, too, but we will get to that later.) However, for an EFI system to operate correctly, the fuel must be maintained at a known pressure and that requires the use of a pressure regulator not unlike the one on the top of your BBQ gas cylinder.

So, now we have two new components to consider. We need a high pressure fuel pump capable of delivering a sufficient volume of fuel to a pressure regulator that can then maintain the flow at the required pressure. A fuel pressure regulator can be a separate, standalone device or can be built into another part of the system. In either case, it will usually contain a spring-loaded diaphragm valve that passes fuel straight through until the required pressure is reached and then holds it there by diverting the excess fuel, via a "return line", back into the tank, to be used later.

Rhapsody has three fuel tanks and that complicates matters somewhat. The "return line", from the pressure regulator has to be able to send the unused fuel back to the tank from which it is drawn. This ensures that messy and dangerous tank overflows don't occur. At its simplest, a dual set of piping, one for feed and one for return, would be required for each tank. Also, a number of manual or solenoid operated taps would be needed to manage, what would be, a fairly complex system.

Solenoid taps are just that - they are ordinary fuel taps that are electrically operated. Each of the three fuel tanks has its own solenoid operated stop tap and each tank also has a low pressure fuel pump to move the fuel to the engine. Only one pair of solenoid taps, with its associated low pressure pump, can be selected at any one time and this is done from a dashboard mounted switch. The solenoid taps are all of the "Normally Closed" type, so that, in the event of an emergency or power failure, all the fuel sources are automatically shut off.

This approach simplifies matters considerably but doesn't solve the problem of returning unused fuel from the pressure regulator to the selected tank. The solution is to have yet another, additional, tank, near the engine, that is filled by the low pressure pump from whichever one of the three fuel tanks is currently selected, and which then also receives the returned fuel from the pressure regulator, rather than trying to send that fuel back to the tank from which it was drawn. Such a tank can be quite small since it is always kept filled by the low pressure pumps.

In times past, in both boats and other vehicles, gravity was used to feed fuel into the engine. This was done by transferring fuel, by hand generally, from the main fuel tank into a small tank, at the right height, on a nearby bulkhead. This was usually done at the beginning of each day's work and the small auxiliary tank became known as a "Day Tank". Also, vehicles being used in a high performance setting may have separate, small, tanks, to overcome fuel starvation caused by extreme cornering forces. In that application it's called a "Surge Tank". Happily, such tanks are readily available via the tuning suppliers, (on eBay - of course), to the vehicle racing fraternity.

So, Rhapsody now has a Day Tank. Its capacity is only 1.25litres and provides fuel, at high pressure, to the regulator (and therefore, the injectors) and then accepts the "returned" fuel to "go around again" without having to return that fuel to the original tank. Now, you may be wondering where the Day Tank gets its high pressure fuel from - it's just a tank, right? Well, no, it is more than that, because it also contains a high pressure fuel pump that is built in.

The high pressure fuel pump is a Bosch Type 044. It's a common aftermarket unit that fits directly into the top face of the Day Tank. At full power, it can provide a fuel flow of 200 l/hr at up to 75psi and draw 11 amps. All of which is gross overkill for this application, but it's nice to be operating well away from its performance limits. One concern is that it may turn out to be noisy, but we'll see.

High pressure pumps and injectors have very fine apertures and tolerances, which means that the fuel has to be very clean. So, a 40 micron, inline filter unit is included in the design. It consists of an aluminium tube with removable ends, so that the internal filter element can be cleaned and/or replaced. It naturally needs to be compatible with the expected pressure and flow rates.

Having assembled all the components for the EFI fuel system, it was surprising how many different sizes and types of connectors are used when, presumably, one type would do. It is epitomised by the old engineer's joke "We love Standards - there are so many to choose from". The solution was to physically take all the component items to Automotive Performance Distributors in Railway Terrace, Mile End, and ask for help. And help they did. They were happy to spend the time to find all the right connectors and adaptors to enable the system to be put together. Thanks, Guys!!

In summary, fuel is delivered to the Day Tank from any one of the three main fuel tanks, via their dedicated low pressure pump and solenoid operated tap combination. The Day Tank and high pressure pump then deliver fuel to the injectors, via the pressure regulator, and also accepts the unused fuel from the Return Line, to "go around again".