Van's Air Force
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DeltaHawk for RV-14
- Thread starter czechsix
- Start date
In Direct Injection engines, the fuel is injected when it is time to combust, at very high pressure (29,000psi for common rail diesels) so atomization is instant. They don't rely on turbulence inside the cylinder to mix the fuel and air like a carburetor or throttle body injected engines. And in a diesel, there is always more air than fuel (until you get to critical altitude) since the air charge is always the same, only the amount of fuel varies, unlike a gas engine that wants the the mixture perfect every time. There just isn't an opportunity for early ignition, if there was, it would ignite early every time.
All the extra air is one of the emission issues diesels have historically had. All that extra air, plus the sulphur in diesel until recent regulator changes, made sulphur dioxide and nitrous oxide emissions high. The reduction of sulphur, plus the use of DEF to break down the nitrous dioxide before it exits the tailpipe have made significant improvements in emissions.
Unburned hydrocarbons happen in diesels when too much fuel is injected, then they start to smoke. When some idiot is 'rolling coal' after they take all the emission control off their squatted diesel pickup, all they are really doing is pouring fuel (and money) out the tailpipe.
Freemasm
Well Known Member
Very much appreciated. Couple of questions/comments if you don’t mind. My experience is in turbine combustors and related emissions. My limited experience looking at DI engines, I don’t recall any of the others injecting at TDC but will admit I wasn’t looking for that detail at the time. Atomization and mixing influence each other but are not the same. You don’t want the fuel particle recombining. Sounds like their engineers have figured it out, at least good enough for our/their applications for now. Wonder if they are or considered injecting in more than one point per cylinder. NOx emissions (NO, NO2, N2O plus radicals, etc) are a family of nasties which is one of the reasons mitigation is so tough. No thing that does benefit across that family is lower flame temps. That plus EGRs, and catalytic converters, in SI engines.
I’ll say again, their UHC emissions are probably not going to be good. I’m relating/expecting this from the 2 stroke approach, not the DI (at TDC for that matter). Any possible NOx benefit from their combustion mixture is probably cancelled out and then some because of the heat of the related combustion event.
My hat is off to their engineers and patient investors.
Much thx for your input.
The fuel may be injected before TDC, but at a given power setting it would always be injected at the same time. The fuel/air charge depends on the pressure and temperature to ignite. A gas engine has a fuel air mixture that can ignite, but is waiting for the sparky thing to set it off, so other things might take the place of the sparky thing and set it off early, causing detonation and kickback if it happens during starting
czechsix
Well Known Member
Based on DH's cutaway animation of the engine it sure looks like the fuel pump cam is firmly bolted to the end of the crankshaft, which implies that the fuel is injected at a fixed timing throughout all RPMs and power settings. I assume modern diesel cars & trucks take advantage of computers to control the timing of the fuel injection depending on RPM and other factors to optimize efficiency...at least that's how conventional gas engines operate. Perhaps DH chose to keep it simple with fixed injection timing and given the limited RPM range that this engine operates in, it provides acceptable performance. Or maybe they have a means of mechanically adjusting the fuel injection timing that I can't tell by looking at the cutaway animation...
czechsix
Well Known Member
Just for kicks, I mocked up an RV-14A with the DHK200 on the nose (using drawings Craig posted earlier for the taildragger). It'll look a bit sleeker once they revise the cowl for the smaller spinner taking advantage of the reduced diameter starter ring gear on the production engine. They were looking at moving the engine/prop back a couple inches as well...
Freemasm
Well Known Member
First off, let me reaffirm that I'm not a piston engine guy. That said, the DH PP does utilize injectors so that timing is ultimately determined by those. I can't tell from the diagrams if there's a fuel return line -> don't know if the upstream injection pressure is relatively constant but guessing no. In keeping with their philosophy of engine operation/electrical independence, I'd assume mechanical injectors. Would be great to get more direct exposure here.
Most mechanically injected diesels are "timed" by the injector pump. The "injector" is just a check valve. The pump works much like a gas engine distributor, rotating it slightly one way or another changes the timing. Looking at the video this system is very similar. But the injector pumps are driven straight off cam lobes. So the timing is set by the cam and not adjustable. It's a simple, solid system, and with the narrow rpm range adjustable "tuning" probably isn't worth the trouble.
Freemasm
Well Known Member
OK. Sorry for not getting it but trying to learn. Each FI pump is a PD pump so ~ same amount of fuel per stroke/crank rev. Still trying to conceptualize how fuel flow/speed/power are controlled in this PP.
A diesel mechanical injection pump is a variable volume (and somewhat fixed pressure) pump. The volume of diesel fuel injected into the combustion chamber for each stroke controlls the power output (and speed) of the engine. The fuel injectors at the cylinders have a "break open" minimum pressure to provide a proper spray patern of the fuel.
Freemasm
Well Known Member
Cool. So the plunger pumps are technically variable displacement (VD) pumps versus PD pumps. This would solve the fuel throttling question. Next dumb question from me would be how the displacement (volume) is varied in the pumps. VD pumps in hydraulic apps are flow controlled by varying the stroke, As that is fixed here being a function of the cam profile, the displacement/volume is changed by some longitudinal variance? Wonder if the opposite pump cylinder end position is varied (can see springs in the video) by some force balance between internal pump pressure, the spring, and an opposite/external pressure; possibly hydraulic (fuel) pressure boosted from a gear type PD pump. A single control/lever would then control this pump output pressure -> that controls fuel pump chamber size -> that controls fuel flow (per engine rev) -> that controls engine speed/power. Much to learn about this seemingly wonderful design.
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Yea, the throttle question isn't clear to me either. Maybe vary the length of the cylinder that the pump piston rides in? I'm picturing a brake master cylinder where the first part of the travel is outside of the cylinder?
Hi
There is nothing simple about these mechanical injector pumps, I found this youtube that explains how a helix pump work. Also found some diagrams of these pumps they also use spinning weights and springs to vary the timing. The mechanical engineers at DH deserve much kudos , remember on their engine they had to design the pump from scratch as it's 4 individual pumps in one body. One for each cylinder / injector.
Quote "Simple is hard"
There is nothing simple about these mechanical injector pumps, I found this youtube that explains how a helix pump work. Also found some diagrams of these pumps they also use spinning weights and springs to vary the timing. The mechanical engineers at DH deserve much kudos , remember on their engine they had to design the pump from scratch as it's 4 individual pumps in one body. One for each cylinder / injector.
Quote "Simple is hard"
Freemasm
Well Known Member
Not arguing as I truly don't know; but, looking at the DH graphics on their website and the previously attached video some (possible) distinctions jump out. The injectors are "remote" from the pumps. In itself, not a real difference. Hydraulics are hydraulics, and proximity usually isn't a consideration unless to an extreme where water hammer could occur.
I'm trying to figure out their system via the aforementioned graphics. One thing that makes the guesses more difficult is each of these engine graphics seem to be a little different in the area of interest.
I'm not saying otherwise but not sure DH utilized Helix valves as described in the associated video, which did a great job of illustrating the operation IMO. Unlike the helix valves, the return lines appear to be on the injector proper vs the pump(s). If that is true, regulation is probably at the injectors. So what is the mechanism that is effecting the individual injectors where the "pairings" are not in proximity to each other? Still guessing some type of hyd pressure control. At least on one of the diagrams, there are four (major, long) lines per side. Two are obviously for fuel delivery. I would assume the larger flexline is a common return for the cylinder pairing. The forth is a smaller hardline which implies (to me) high pressure, low flow. It also comes from the accessory area of the engine.
A force balance between spring, fuel pressure, and hyd (fuel as medium at control pressure) is still my best guess; fuel flow being controlled by directly varying the subject hyd press.
Still cool. Wish I had facts vs. guesses but haven't found any real details yet.
Have you got a link to the graphic on the DH web site? Also a lot of the novel features of the DH engine are covered by patents that are published, if you take a look at https://patents.google.com/patent/US20050235946A1/en section paragraphs 0010 & 0011 it talks about how they use the excess return fuel to cool the injectors. If you suffer from insomnia you can read through all the patents
