Postby vvega » Sun Mar 22, 2009 10:27 pm
LPG Fumigation
Page Four
Calculating the amount of gas to apply:
For the simple dump system on the Pusher, I calculated gas flow in BTU's, since this was about the only way that I could understand how to correlate diesel-vs.-LPG. Here's what I started with:
The Pusher, using a 1.6 liter diesel engine gets around 30 MPG of diesel fuel. A gallon of diesel contains 128,000 BTU's of thermal power. Assuming 60 MPH, the consumption would be 2 gallons per hour, or 256,000 BTU/hr. I was aiming at 10% LPG for a start, so I simply divided 256,000 by 10 = 25,000 BTU of LPG.
Consulting the chart at in the appendix of this section, we see that in order to get 25,000 BTU/hr from the regulator that I planned on using (11" WC), I'd need an orifice of 1.5mm diameter, or a # 53 number drill bit.
Calculating for an ordinary VW Rabbit, which gets 50 MPG, we see that fuel consumption is 1.2 gallons per hour at 60 MPH = 153,600 BTU. 10% would be 15,400 BTU, or about 1.2mm/#56 bit. Using the larger orifice from the Pusher would yield about 16% LPG.
Determining gas rates for turbocharged engines is a bit more troublesome, as there is a lot more air available to add fuel and gas to. What I have been told is that gas pressures can run in the 6 PSI range, with orifices of ¼ to 3/8 of one inch. This is a *lot* of gas, and will require a regulator capable of the increased flow, as well as a much larger storage tank for the gas. See the gas flow calculator in the appendix of this page.
Tuning it up:
Adjusting the orifice and/or gas flow for optimum performance could be tricky. Basically, increase the LPG flow incrementally until you see/hear detrimental effects. The first, and most noticeable is hard knocking, or pinging from the engine. If it sounds like someone threw a handful of ball bearings inside your motor when your turn on the gas, by all means TURN IT DOWN!!! The rattling you are hearing is the excessive peak pressure, and is also the sound of your pistons being turned into molten aluminum. If anything, adding LPG to a diesel engine's intake air should make the engine quieter.
Adding too much LPG can result in your diesel engine's timing being thrown for a loop. The normal injection window of fuel delivery of a diesel engine is that the fuel is metered into the combustion cylinder (or the pre-chamber if it's indirectly injected) in a controlled manner, which is calculated either mechanically or electronically to correspond to the position of the piston during the power stroke. A excess of LPG will upset this fuel-to-piston timing. A combustible air-to-fuel ratio of LPG will be ignited by the very first introduction of diesel to the superheated combustion charge. (Remember, LPG/air mixtures should not normally be ignited by compression temperatures alone. The first atomization of diesel into the cylinder will act as a "spark", setting off the LPG/air mixture.) The flame front of the LPG will contribute to cylinder pressures, and the remaining diesel injected during the injection window will cause the peak pressures inside the cylinder to be overly high, resulting in the "pinging" you hear. This should be avoided at all costs!
Another thing to watch for if your engine has a pyrometer (exhaust gas temperature gauge), is EGT's dropping. LPG promotes more complete combustion, so some of the heat that used to escape through the exhaust pipe is now being converted into mechanical power and transferred to the wheels.
Diesel engines do not react well to LPG fumigation when they are cold. Turning on the gas before the engine has warmed properly will result in rough idle and bogging, or lack of pulling power. Let the engine come up to something near operating temperature before turning on the gas. Another reason to have a dash-mounted control switch.
Exhaust Gas Temperatures:
Before you begin fooling around with any aspect of diesel performance, you should already know the design limits of EGT's for your engine, and have a method of measuring them accurately. This is usually accomplished by the addition of a pyrometer to the exhaust manifold so that the temperature of the exhaust gas can be monitored. EGT's are the best and most accurate method of monitoring the load on a diesel engine, and can be used along with observation of exhaust opacity to determine effects of changes to fueling, timing, breathing, LPG quantities, and other performance modifications to your engine.
The best place for the thermocouple that detects the exhaust temperature on a TD engine is pre-turbo. This will give the fastest response and the most accurate reading. In some cases, it may not be possible to put the thermocouple before the turbine, in which case the reading on the gauge must be corrected by a small factor to allow for the cooling effects of the turbo. Adding 300-400 degrees Fahrenheit to the reading should get you back to the actual temperature of the exhaust.
There are many suppliers of EGT gauges, as well as many manufacturers such as Isspro. Also don't neglect that pyrometers are common in aviation, and gauges used on airplanes are likely to be more rugged and accurate than those used in automotive applications. A good source for used EGT gauges might be your local heavy truck wrecking yard.
What to Expect:
If my experiences using LPG fumigation on a normally-aspirated VW diesel engine are any indication, don't expect a lot, and you won't be disappointed. I don't get any huge surge in power, in fact, it's unnoticeable for the most part. If I'm holding a steady speed and turn on the gas at, say, 60 MPH, I see a gradual increase in speed of about 4 MPH. Someone with better math skills than me calculated that accounting for wind and rolling resistance, the increase in power was about 21%. Since my EV and Pusher are heavy (4,500 pounds, together), and the diesel engine puts out a paltry 52 horsepower, even a 20% increase in power is welcome for passing and climbing mountain passes.
Turbocharged engines will realize a dramatic increase in power when fed an adequate quantity of gas. I've heard stories of long-haul trucks using fumigation burning up transmissions and melting tires after storming up a long steep grade, the rest of the truck's drive train wasn't adequate to handle the power boost the LPG provided. Of course, tweaking the boost pressure, installing a modified engine control management computer chip, and providing intake and exhaust flow enhancements are all a part of race-prep for diesel engines. If you have a turbocharged vehicle, you are probably going to want to look into a commercially manufactured fumigation system to make sure that you get the most from your engine, with the least likelihood of engine damage.
Notes about air flow
Probably the single most important thing you can do for a diesel is to improve the air flow into and out of the engine. This is particularly important in a N/A engine.
Intake Air:
First and foremost, remove any and all possible flow restrictions. Several low-restriction air filters are available, such as the K&N brand filters. Increasing the diameter of the intake ducting is also important. On the VW diesel engines, the "snorkel" which leads into the air filter box is designed to minimize intake noise ~not~ maximize flow. Converting to a larger pipe will mean that the engine has less pulling loss, resulting in more air per piston intake stroke, which means you can stuff more fuel (and/or LPG) into the engine, resulting in more power. Check out the 3" ducting on my engine in the photo above in the "Supplying gas to the Intake" paragraph above. This ducting is fed by a cowl scoop at the back of the hood. I like to think that it at least decreases intake air restriction by allowing the outside air to be a positive pressure.
Temperature of the air is also important. Cooler is better. Cool air is more dense, more oxygen to aid in combustion. The intake end of the pipe leading into the air filter should be supplied with the coolest air possible. Usually, this means from either the grille, a hood scoop or under the front bumper of the car. What you don't want is air that has been warmed after coming through the radiator, in other words, engine compartment temperature air.
Turbo engines benefit from installation of an intercooler, which is essentially an air-to-air heat exchanger that removes the heat produced when the intake air is compressed by the turbine. Cooler boost air will allow greater gains in power through increased fueling, either through adjustment of the injection pump, by fumigation, or both.
Exhaust Gases:
Getting rid of exhaust gases quickly is also very important. Diesel engines do not benefit from controlled back pressure like gasoline engine do. In all cases, larger diameter exhaust pipes are better. Low restriction exhaust manifolds, down pipes and mufflers all add to power and performance, in dramatic ways. Less exhaust restriction is also beneficial to keeping exhaust gas temperatures lower.
If you are trying for ultimate power, a good old fashioned cylinder head port-n-polish of the intake and exhaust ports works wonders.
Other Mods:
Most anything you do to enhance performance will result in the engine running outside of it's design parameters. It may be necessary to compensate for this by adding high performance parts such as racing head bolts/studs, ceramic-coated pistons, additional cooling system enhancements, larger clutch disc and pressure plate, and even a locking differential transaxle to minimize wheel spin. It all depends on how far you intend to go in your diesel power quest. Like anything, the only limit is what your wallet can support!
Kiwi4x4
