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.

 

[RodRico]

Is there any type of inline 12V, high-precision solenoid valve capable of handling the pressure while at the same time having the necessary open/shut characteristics to produce the very short pulses with the necessary precision?

It's not clear to me what you're planning, but modern diesel injectors routinely control fuel above 20,000 psi. You're flowing a miniscule amount of fuel out of the injector, likely far less than would move through a 0.050 inch orifice at 650 psi, and the force required to hold a 0.050 inch plunger closed against 650 psi is under 2 pounds. Don't try to control flow to the injector, control flow to the fuel return so you can dump injector pressure by opening the return valve. Fuel is incompressible, so pressure to the injector will rise very quickly once the slack in the pump is taken up. Hold the relief port open until you're confident the slack is taken up then use a fast actuator to close the return for the desired time. Modern diesel injectors use piezo stack actuators because they're fast and provide a lot more force than a solenoid. Their displacement is measured in microns, so you need a larger diameter return port to attain a given flow, and that increases the pressure that must be overcome, but piezo stacks produce a lot of force.

 

[moon161]

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

6th button to the left of the Preview button

 

[moon161]

Just drop an OM617 in ;-)

 

[GregLocock]

Yeah my immediate answr was crate engine plus megasquirt. Mainly to give the purists a heart attack. Seriously if you've got a million dollar car, 20k for a fuel pump rebuild is just a good investment.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Do what a friend's high school shop teacher did with his Mopar muscle car (I forget which of the high-end engines it had)

... Took out the high-power expensive engine and stuck it on the shelf (properly prepared for long-term storage, of course), and dropped in a slant six. That way he could drive the car and not worry about it.

That saved him a ticket, too. Someone pulled up beside him at a stop light in some other muscle car, then peeled away when the light turned green ... the cop some distance behind who heard this but I suppose didn't actually see it, pulled over the car that he got to first, which was his (/6, remember). While the cop was yelling and screaming about driving like a madman, he opened the hood to reveal the leaning tower of power beneath. The cop changed his tune. "Yeah, I suppose it wasn't you."

 

[gruntguru]

Yeah - million dollar collector's car - needs to be kept original. If the owner can't afford to keep it original he should sell it.

Even Gordon Murray sold his McLaren F1 once its value got ridiculous. Insurance costs and the risk taken every time he took his friends for a squirt couldn't be justified (even though he could aford it.)

je suis charlie

 

[moon161]

^ This sort of thing is beginning to happen to Mercedes W123 cars like I have. By no means a gull-wing, but the market is getting fancy. 10 years ago the top of the day to day market was a blue 81 300TD with a factory replacement OM617a once owned by Elvira the late nite horror movie hostess for about $12k. Since then, clean or well flipped 300TD's are in the high 20's or mid 30's on Bringatrailer.com. People see more money in parting a car for restoration folks than doing some body work.