Mercedes 300 Gullwing - injection pump upgrade

[Per70]

If this thread is comnpletely out of context here, I apologize and if so - maybe you can point me in the right direction.

Background
Anyway - I'm working on a Mercedes 300 SL -55 (coupe i.e. "gullwing") equipped with a Bosch mechanical direct injection system. The system is commonly considered an engineering piece of marvel for its time, but it also comes with some problems intrinsic to its design. Extreme mechanical tolerances are reqiured to ensure that delivered fuel quantity to each cylinder is consistent and within specs throughout the rev range. When well cared for it works amazingly well, but ultimately - no matter how rigorously maintained - the system will require a major overhaul. Today, this can only be done by a handful of companies worldwide, and some may say that it can only be done properly by Bosch's own classic department. Needless to say, the cost for a complete overhaul is substantial and can easily set you back 20 kUSD or more. This is the problem I wish to address.

I want to put the newly restored injection pump on a shelf (so that it can be put back in the car at any time) and replace it with an upgraded unit using modern technology, while preserving all other components of the injection system. An upgraded unit for regular driving and use while the original unit will be kept for show, is the idea.

Design idea
- A new housing is milled from billet aluminium so that it fits right into the position of the original pump.
- The drive shaft connecting the pump to the crankshaft that used to drive the plungers in the original pump will instead be used to generate a tach signal and a cam-position signal. It will also have a single lobe to drive a high pressure fuel pump (similar to the original pump).
- Plungers will be replaced with solenoid valves, controlled by a custom made ECU.
- The original vacuum line would be connected to a MAP-sensor in the ECU that would take over the role of the diaphragm and control rod in the original pump.
- Rather than trying to synchronize the pressure pulses from the high pressure pump with the injector squirts, I'm thinking that the system could operate with constant pressure instead? The original mechanical injectors will be used and they squirt at an opening presure of about 650 PSI. If the system can produce a constant presure of 650 PSI, then the injected fuel quantity could be controlled entirely by the length of a pulse openining and closing a solenoid connected to each injector.

Questions
I'd be very interested to receive some comments on this idea in general. There's a ton of more info I can share, but the main idea should be quite clear I think. More specifically, I'm asking:
- How can I generate a constant fuel pressure of 650 PSI and even out the pulses created by the high pressure pump? Control valve + accumulator?
- What kind of solenoid could be used? Ideally I'd need something like a common low pressure fuel injector, but without the spray nozzle since the output from the solenoid valve will not squirt directly into the engine, but rather be feed an injector line. It also needs to be able to handle 650 PSI pressure.

Any thoughts on this would be welcome.

 

[TugboatEng]

Just swap out the mechanical injectors for some electronic units and go EFI with the thing. If you want to maintain the appearance of the original engine you can use a gutted mechanical pump housing as the fuel rail and leave all of the original plumbing in place.

 

[Per70]

Sure, but that's not what I'm looking for.

 

[BrianPetersen]

That engine was direct-injection, was it not?

If it's DI, the only modern solenoid operated injectors that will function are modern direct-injectors designed for a modern common-rail (Gasoline direct-injection) system.

This won't be simple, and the cost of getting it all to work properly will probably be more than that of getting M-B to refurbish the original system.

The other choice is to remove the entire intake system and build a new intake manifold and run the engine with a carb.

 

[TugboatEng]

The Bosch pump does just appear to be an adaptation of their diesel pump with some various bellows and solenoids to control the stroke of the pump.

I think your most reasonable option would be to install the parent diesel pump or remove the mechanical controls from a donor gas pump and do all of that tricky mechanical calculation with a standalone electronic controller. You could even adapt a less expensive pump like a Stanadyne DB series but that would require modifying fuel lines.

 

[BrianPetersen]

Thinking about this some more.

Maintaining the original direct-injection concept so that you can use the original intake manifold and cylinder head without modifications so that it looks original-ish means using modern direct-injectors from a modern engine in place of the original "dumb" nozzles.

Let's assume for the moment that some sort of adapter can be made so that this can be done without modifying the cylinder head. Modern direct-injectors tend to have a pretty long tip so that they can get down into the combustion chamber while reaching in between the valve mechanisms and ports of a modern engine, so perhaps this is possible. I can't find a picture, nevermind a dimensional drawing, of what the original injector nozzles look like, so this is on you. Let's suppose you get through that.

Now, obtain the high-pressure fuel pump from a modern six cylinder direct-injection gasoline engine as a complete assembly, and find a way to adapt that to be driven from the drive shaft of the M-B engine. Making high-pressure fuel lines and fuel rail to get from that pump to the injectors is on you. Also obtain the fuel-pressure regulator and the like, assuming it is not built into the fuel pump.

That fuel pump will have the pressure-pulsations situation dealt with internally. You might need to "sync" the common-rail pump so that its single plunger is on its delivery stroke three times per crankshaft revolution together with one of the six intake strokes - I don't know if this is necessary.

The next challenge will be to see if the electrical characteristics of those injectors are compatible with, let's say, an aftermarket Megasquirt fuel injection controller. I have no idea if they are even in the ballpark. More stuff that's on you.

You can expect the direct-injectors to want significantly shorter on-times than a conventional multipoint EFI system wants.

Then calibrate the Megasquirt ECU so that it all works.

 

[BrianPetersen]

I found this. I suspect the voltage and current requirements are well beyond what a normal EFI circuit-board will operate.

https://www.youtube.com/watch?app=desktop&v=4FG8tSWrCl8

 

[PJGD]

I think that with your plan, you are giving yourself a huge amount of unnecessary work and problems. Essentially the Bosch petrol injection (PI) pump is a relatively minor development of the diesel fuel pumps of that era, and the manufacturing tolerances are of a similar level of stringency, thus any competent and sympathetic diesel pump repair shop should be able to renovate this pump. If you disagree with this statement, then please explain why you believe it to be incorrect.

The modifications relative to the standard diesel injection pump will be focused on addressing the issues created by reduced viscosity and lubricity of petrol vs diesel fuel; these include greater leakage past the plungers (resolvable - see attached file) and greater propensity for plunger galling and seizure. Modern pumps intended for low lubricity fuels such as gasoline, and even more challenging Dimethyl-Ether (DME) a very good diesel fuel, use diamond-like carbon coated (DLC) plungers to achieve the durability targets, and is something you should consider.

I am moderately confident that the Bosch PI system used outward-opening poppet nozzles, and these are typically very robust and reliable unless the valve seat is scored up due to dirt and debris passing through. If replacement nozzles are required, I suggest looking at the valve module of the Lucas Microjector as used in the [much maligned] Oldsmobile diesel engine of the 1980's; they typically outlasted the car.

If you are set on the idea of using a modern injection system, then I suggest you consider the Delphi diesel common rail system as used for example on the Renault K9K engine or the Ford Puma. Later developments of this system have been widely used in the Gasoline Compression Ignition research activities. You would need to re-engineer the injector for the outward opening nozzle.

PJGD

 

[BigClive]

If someone can afford to buy a 300Sl for quite a few million dollars I would have thought they would be able to afford to get the pump fixed by Bosch.

 

[TugboatEng]

Maybe this one was inherited? Anyways, it's quite simple to simulate a mechanical computer using an electronic one. This fuel pump has only one control input to position the rack. The position of the rack appears to simple computation based on throttle position and manifold pressure. There are solenoids to change the lower limits based on engine temperature and such.

 

[Per70]

Thanks for all the replies.

Just to be clear - many of these cars have been with their current owners for decades and long before the prices went crazy, so it's unfair to assume that every owner has spent "a few million dollars" buying one.

I have done two Megasquirt projects in the past and if it was only about swapping the existing mechanical injection for an EFI, I would just use MS with common manifold injectors. In this case however, the whole idea is to make the installation indistinguishable from the original unless you're a real expert.
These guys are doing something similar for ignition distributors - a drop in replacement for the mechanical unit.

http://www.123ignitionshop.com

I am an experienced microchip programmer and I have made my own modifications to the Bowling/Grippo MS firmwatre, so the ECU programming/tuning is not an issue here at all. I also have free access to a CNC machine so I can fabrigate a new pump housing that's externally identical to the existing one.

The key issue is about sourcing the neccessary parts for fuel pressure and fuel delivery control to the existing injectors, or more specifically:

- How do I secure a constant and steady 650+ PSI fuel pressure?
- Is there any type of inline 12V, high-precision solenoid valve capable of handling the pressure while at the same time having the neccessary open/shut characteristics to produce the very short pulses with the necessary precision? These solenoids would be connected to the "fuel rail" in one end and to the injector line in the other end so an atomizing nozzle won't work. Maybe there isn't any solution for this and in such case the project will be abandoned.

 

[Per70]

@PJGD - many thanks for your input and the attached doc. I have had the same reasoning with a number of diesel service firms but nobody wants to touch this pump. I know that the calibration protocol is more extensive for the petrol pumps, but I cannot give you a good explanation why it's considered to be so complicated. All I know from extensive research is that finding a a service shop with a track record of restoring these pumps is extremely difficult.

Coating the plungers is a very interesting thought since whatever the outcome of the pump upgrade project, the original pump will be overhauled. Are there companies who can do this as an "aftermarket" service or is it exclusively for manufacturing of new parts? I assume the coating will affect the tolerances as well so presumably the whole plunger assembly would require relapping?

 

[SwinnyGG]

Coating the plungers is a very interesting thought since whatever the outcome of the pump upgrade project, the original pump will be overhauled. Are there companies who can do this as an "aftermarket" service or is it exclusively for manufacturing of new parts? I assume the coating will affect the tolerances as well so presumably the whole plunger assembly would require relapping?

I suspect you could coat them without needing to modify anything- there are commercially available DLC coatings as thin as 0.5 micron. I know injection pump parts have high tolerances but I don't believe they are that high.

 

[BrianPetersen]

I already answered these questions, but let's try again:

- How do I secure a constant and steady 650+ PSI fuel pressure?

With an off-the-shelf high-pressure fuel pump (HPFP) from any production GDI (gasoline direct injection) engine of your choice - and just about everyone has direct-injection engines in production nowadays. There are lots of different shapes and arrangements (and prices). Google-search "gdi pump" to get started.

Your proposed injection pressure is about 44 bar. Typical GDI injection pressures nowadays are in the 200 bar range.

- Is there any type of inline 12V, high-precision solenoid valve capable of handling the pressure while at the same time having the neccessary open/shut characteristics to produce the very short pulses with the necessary precision? These solenoids would be connected to the "fuel rail" in one end and to the injector line in the other end so an atomizing nozzle won't work. Maybe there isn't any solution for this and in such case the project will be abandoned.

With complete off-the-shelf injectors for any such GDI engine. It's not just a "solenoid valve" - it is a complete injector which will completely replace the injectors that you already have in the engine. Up to you to figure out how to adapt one to the engine. Forget about trying to use the injectors that are already there with a separate solenoid valve, that won't work. Forget about trying to use solenoids that are remotely mounted from the injectors, that won't work either. These need to open and shut within a few milliseconds with very precise control. "Dead volume" between your proposed separately-mounted valves will be killer. The way modern GDI engines do it - with an integrated solenoid-operated injector - is the way it has to be done.

You are going to end up with an externally-visible wiring harness from each injector to wherever you are putting the ECU.

 

[TugboatEng]

It's not the pump that wears out over the life of the engine, it's all of the things attached to the pump. During injection pump remanufacturing, all of the actual pump parts get reused and it's typically only seals getting replaced.

That's why I'm saying, keep your current pump and electrify the control system.

 

[Per70]

Well, I already responded I think, but let's try again: I don't want to replace the injectors - I want to keep the existing ones. I do get your suggestion, but that's not an alternative.
What do you mean by "dead volume" will be a killer? It is exactly how the the original setup works. The plunger generates a pressure pulse in the injection line causing the injector to open when this pressure exceeds a certain value, squirt the volume fed by the pump plunger, and then close as the pressure drops below the opening pressure. That's all there is to it. All I want to do is to generate this pressure pulse in a different, electronically controlled way.

I didn't state that a fuel injector is "just a solenoid valve", but - again - I'm not asking about fuel injectors, I'm asking about an inline solenoid valve or any other existing piece of equipment that I may not be aware of that can help me solve my specific problem.

 

[Per70]

@TugboatEng -
agree that this is the common way, but in this case it is actually the plungers and their sleeves that ultimately need to be replaced. In the early models of the these pumps there are no seals to be replaced.

 

[SwinnyGG]

I'm asking about an inline solenoid valve or any other existing piece of equipment that I may not be aware of that can help me solve my specific problem.

There certainly are small, in-line solenoid valves which can handle the pressures, flows, and rapid switching that this application requires- they are contained within every modern direct injection fuel injector. You could potentially adapt a DI injector and use it as an in line solenoid valve (by using the valving gear only and removing the atomization components which are downstream) but doing so in a way that is safe (don't forget... keeping fuel contained at 200 bar is no small feat of engineering, and the consequences of failure are extremely high) and will be reliable and programmable is a very serious engineering challenge.

I always hate to tell people that what they want to accomplish isn't possible, but unless you have a very significant engineering budget to throw at this, or hundreds (thousands) of hours of your own time, what you're after isn't possible. It certainly won't be possible by connecting a bunch of off-the-shelf components without having to modify them significantly.

 

[TugboatEng]

If it's just the plungers and sleeves that need to be replaced, anybody can do that and it requires no great technical skills or special tools.
There are seals in the pump. The seals are for the housing and throttle linkages. It's these seals that fail allowing the pump to drain out when it sits causing hard starting.

 

[Per70]

Sorry, can't figure out the editor here. How do you include a quote from a previous post?