Impco LPG carburetion theory and operation

Carburetor. IMPCO carburetors are an air-valve design, using a relatively constant low pressure drop to draw fuel into the carburetor from cranking to full load. The advantage of this type of construction is a strong "signal" or vacuum set up by a metering spring (A) holding the air valve (B) closed. A pressure drop (C) under the air valve of approximately 6" WC (Water Column) is required to open the valve during cranking. Vacuum to the converter (D) and filter/fuelock (D) allow fuel to flow. When the engine is stopped, fuel is safely sealed off within the carburetor, converter and fuellock.

The air-fuel metering upper part of the carburetor, called a mixer, is completely self-contained. It derives it's vacuum feedback signal from the throttle bore.

Converter. The IMPCO converter is a combined two-stage regulator and vaporizer. It receives liquid fuel at tank pressure (approximately 180PSI) from the filter/fuelock (E) and reduces pressure in two stages to slightly less than atmospheric (F). When the engine is cranking or running, a partial vacuum is created in the vapor line to the carburetor (D), which opens the regulator permitting fuel to flow to the carburetor.

In the process of reducing the pressure in the tank to atmospheric, the liquid propane expands to become a vapor, absorbing heat in the process. Water from the engine cooling system circulates through a heat exchanger (G) to avoid freezeup. The regulator seals off fuel flow when the engine is stopped (H).

IMPCO makes three sizes of converter, the Model J which is rated at 100HP maximum, and the Model L and Model E which are rated at 325 HP maximum.

Fuelock/Filter. The filter/fuelock (Model VFF30) is vacuum operated. Upon slight engine vacuum (2" WC), it opens and allows fuel from the tank to flow through a fuel filter, to the converter. Any time the engine stops, whether the ignition key is on or off, the fuel is shut off automatically. It also filters any large particles of solid material out of the fuel before they reach the high-pressure side of the converter.

Diaphragm-operated Air-Gas valve carburetion

IMPCO carburetors use a flexible diaphragm made of fiberglass and synthetic or silicone rubber, except the Model 50 which uses a piston and O-ring to reduce size for small engines. Both work in the same manner however. Cranking or running the engine produces a vacuum in the throttle body. Through passages in the air valve (P), the pressure on the top side of the diaphragm (D) is lowered; atmospheric pressure on the lower side of the diaphragm pushes upward against the metering spring (S).

At approximately 0.2PSI (6" WC), the air valve lifts off its seat. At approximately 0.5PSI (13.8" WC) the air valve is in the fully-opened position.

The lowered pressure on the top of the diahragm varies with engine speed and throttle position (T). The air valve therefore measures the air flow through the engine by responding exactly to engine speed and throttle position.

The controlled pressure drop of 0.2 to 0.5PSI setup by the metering spring provides the signal necessary to drawn fuel into the air stream within the mixer. The gas metering valve (V) is mounted on the bottom of the diaphragm, and is shaped to proportion the fuel/air mixture for the air valve position.

IMPCO carburators have two mixture adjustments:

1. Idle mixture bypass (I). Due to the air vavle design, the total volume of air/fuel passing through the throttle is constant; increased idle bleed (turn screw out) introduces air into the mixer, and the air valve closes slightly, thereby lowering the amount of fuel in the fuel/air mixture.

2. Power mixture adjustment (A). This effect air/fuel mixture only when the air valve is nearly or fully open; it then limits air/fuel ratio by effectively restricting inward fuel flow. It can be set only under near-full-load conditions.

Mixtures between idle and full-load are determined by the shape of the gas metering valve. It is shaped to produce lean mixtures at low loads and richer mixtures at high speeds and hevy load.

The shape of the gas valve is designed for optimum mixture for the middle of the carburetor's design range.

Selecting correct carburetor size

Air-flow capacity: It is important to correctly size the carburetor to the engine's operating air-flow requirements. Too small a carburetor limits horsepower; up to some specific speed, normal torque is obtained. Beyond that point air-flow is limited by the carburetor and power drops off.

A too-large carburetor will cause starting troubles (too low vacuum signal at cranking), idle will be hard or impossible to set, and fuel/air mixture will be incorrect.

However, type of service also has a bearing on carburetor sizing; an engine that is never operated at full-load conditions is sometimes better off with a slightly-small carburetor; it will have improved low-end torque and easier starting. Passenger cars, forklifts, etc are good examples. Engines operated at or near full load, such as stationary engines, generators and the like, may produce better power with a slightly oversized carburetor.

Determining engine air-flow from the charts: The chart below gives air-flow requirements for most engine sizes. Use the next-closest value. Engine RPM should be the highest speed during actual use, such as accelerating under full load at highway speeds.

Calculating exact engine air-flow: You can easily calculate the actual engine air-flow directly. For normally-aspirated engines (ie. not turbo- or super-charged):

CID * RPM / 3456 * 0.85 = CFM required

CID is engine size, in cubic inches (CID = cubic centimeters (cc) * 0.06102), RPM is maximum engine speed. 0.85 is a close approximation of Volumetric Efficiency.

For turbocharged or supercharged engines:

CID * RPM / 3456 * %boost + 1.00 = CFM required

Normal inlet air pressure is 14.7PSI; supercharging merely increases inlet pressure. For example, 6PSI boost means 20.7PSI pressure, or 140% boost.

IMPCO Carburetor models vs. air flow

Vehicle applications

	IMPCO		Max.
	Model		CFM
	   50		 91
       50-500		108
	  100		170
	  125		202
	  175		210
	  200		276
	  225		329
  300A-1, -20		348
 300A-50, -70		432
	  425		460

Industrial/stationary applications

	IMPCO		Max.
	Model		CFM
	   50		118
       50-500		124
	  100		197
	  125		235
	  200		345
	  225		380
	 200D		468
	  425		533
	 200T		680
	 600D	       1600

Engine size and speed vs. air flow

NOTES: These tables assume 85% VE as mentioned in the text. To convert liters to cubic inches, multiply by 61.02. For two-cycle engines, double the CFM value found.

Engine speed, RPM  400 - 2400 RPM
   CID	 400  600  800 1000 1200 1400 1600 1800 2000 2200 2400
    50	   5    7   10   12   15   17   20   22   25   27   30
   100    10   14   18   23   28   33   37   42   46   51   56
   150    14   21   28   35   42   49   56   63   70   77   84
   200    19   28   37   46   56   65   74   84   93  102  110
   250    23   35   47   58   70   78   93  105  116  128  139
   300    28   42   56   70   84   98  112  126  140  154  168
   350    32   49   65   81   98  114  130  146  162  178  195
   400    37   56   74   93  111  130  148  167  185  204  223
   450    42   63   83  109  129  149  169  189  209  230  251

Engine speed, RPM  2600 - 4000 RPM
   CID	2600 2800 3000 3200 3400 3600 3800 4000
    50	  32   34   37   39   42   44   47   49
   100    60   65   70   75   79   84   88   93 
   150    91   98  104  112  118  125  132  139
   200   121  130  139  148  158  167  177  185
   250   151  163  174  186  198  209  220  232
   300   182  196  208  224  236  250  264  278
   350   212  228  244  260  276  293  309  325
   400   242  261  280  298  317  335  254  372
   450   272  293  314  335  357  378  399  419

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