intake air heater for diesel engine

The following request for suggestions comes from an engineering school dropout and experienced hobbyist mechanic. I'd like to make an air heater for the intake of a diesel engine
to help with starting in extremely cold weather. Propane will provide the heat. I have an idea of fabricating something similar in principle to the familiar Modine heaters that hang from the ceilings of many light industrial loft spaces. I.e., the burning propane will go through channels that intersect the air intake, making a heat exhchanger that won't pull any of the burnt propane into the engine. The engine's air intake configuration features one section of 3" O.D. metal pipe about 18" long, clamped to a rubber hose at each end. I plan to retire that piece and install, in its place, an assembly fabricated from standard diesel exhaust piping, adapters and other cheap parts. I'll use a section of 3.5" exhaust pipe and connectors that neck down to 3" to connect to the existing hoses. I'll conserve the existing expensive engine part and can happlily hack away at those cheap parts in attempting to fabricate a heat exchanger, without fear of repercussions. Ideas: Cut into the 3.5" pipe and weld or braze in some pieces of metal tubing or something of the sort, to provide channels for the burning propane and transfer heat to the intake air. Build an inner chamber where the propane will combust. If that looks too much like work, maybe just flatten a spot on the pipe, bolt a heavy aluminum heatsink inside and heat the flat spot on the pipe with a propane flame from the outside.
I estimate that, even at idle, I'll need to add on the order of 10,000 BTU/hr to the intake air to make this engine feel really, really happy with the block at zero fahrenheit. Sort of a not-very- well-informed guess. 5.9 liter engine. No, I can't use a block heater. I live off the grid.
So the question is, what's the best way for a guy with basic tools to make such a heat exchanger? Angle grinder and a gas welding setup, stuff like that.
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Dear gearhead:

...
Preheat the fuel instead. Air carries too little heat per degree.
David A. Smith
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First a few numbers: A 5.9 Liter engine at 1000 RPM sucks in about 600 ft^3 per minute. If you want the intake at 300f, that's about 30# of air per minute. The specific heat of air is .24 BTU/# so you need about 2,000 BTU per minute.
I would build a "fire tube" style heater. In this type heater the flame is carried thru the main heater in relatively thin tubes.
I would make it this way: a. 10 lengths of 2" dia. muffler tube, 3" long. b. 10 pieces of 1/8" thick x 3" long by 2.5" wide hard board. c. 9 pieces of 1/8" x 2" long x 3" wide hard board. d. 2 piece of scrap metal 2" x 3" x 1/2" thick.
Take "a" and squash it in the middle with "d", using "b" to keep the inside open.
___ ___ \_________________/ _________________ ___/ \___
Repeat 9 more times.
Stack them all together, using "c" to help align & space them. Bend to suit, weld the open ends of the tubes together. Fab the end bells (air in & out) from 3.5" tubing. NOTE: this will leave two hot sides exposed to the outside. Fab flame box out of 3.5" tubing so it feeds flame down thru the squashed tubes.
I'd probably try a Bernzomatic JT680 torch head to start, just to proof the heat quantity.
Dave J.
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wrote:

Yes, having the intake air go through the tubes, surrounded by a box of flame, works best. For some reason I had the idea of doing it the other way around... I wonder if there are any small fire tube heaters that can be had for salvage. By the way how did you get 600 ft^3 /min. Is that an industry figure? When I took a naive theoretical approach, going by displacement, I got a much smaller number. 5.9 liter = 360 in^3. Half the cylinders draw air on each revolution, which comes to 180000 in^3 or 104 ft^3/min. I'll look into the JT680. Some of the torches marketed for melting ice on driveways etc. are, in effect, flamethrowers. A friend of mine bought one. I'll have to go over and ask him to let me play with it. Just what I need under the hood of my truck ;)
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Obviously I didn't read your post very carefully. You state clearly to have the flame going through the squashed tubes.
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CFM I converted 5.9 liters to .208 cubic feet. Then screwed up, by multiplying by 5.9 again,( thinking that it was Liters / ft^3). So, you are correct in the volume, and need to reduce the heat accordingly. My apologies.
Still think I would make the heat exchanger that way.
Dave J.
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wrote:

Well, even at 100 cubic feet per min, the air will be going through there so fast that I think a small firebox like I envisioned, just a few inches in diameter, might not be adequate. It might take some tens of cubic feet of air volume in the box for the moving intake air to linger long enough to absorb much heat, depending on how well I can build it. And it can't impose too much restriction on the flow of intake air, because at some point the engine has to run normally. In other words, I'm looking at a _big_ heat exchanger. My truck has quite a bit of room under the hood, but I'm not sure there's enough for that. The truck gets its air from a cavity in the front fender, which might have enough room to install a firebox, as well as being safer than doing it underhood. But then the heated air has to go through the air filter, turbo and intercooler before reaching the intake.
Heating the intake air just looks impractical. Better off heating the engine coolant, the way a Webasto parking heater does. And those things cost like a kilobuck. So I still have motivation to undertake a project... a homemade "parking heater."
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Well, a radiator does a pretty good job, in just a few inches, but that's your call.
As an option, perhaps a Diesel powered hydronic coolant heater. They need a little 12V, but use diesel or gas for fuel. http://www.espar.com/html/products/coolantheaters.html http://www.webastoshowroom.com/blueheat/blueheat_products_ttc1.htm
Of course, I would try a DIY. Maybe using Kerosine or WVO.
Dave J.
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