Why Alternate Fuels?
FAA facilities use propane and natural gas instead of gasoline because underground gasoline storage tanks created environmental contamination risks. Above-ground alternative fuel systems make leaks easier to detect and address.
Part 1
Foundation Concepts
These are the concepts the companion establishes before any component-level troubleshooting starts: why alternate fuels are used, how the schematics are split, how WC works, and how the Magnehelic gauge must be connected.
The Two Systems
| System | Schematic | Designation |
|---|---|---|
| Propane | Figure C-1 | Most common FAA lesson configuration |
| Natural Gas | Figure C-5 | Utility-supplied configuration |
How the Schematics Are Read
Each system is split into two sheets. Sheet 1 is the downstream side near combustion. Sheet 2 is the upstream side where fuel originates and is prepared for delivery. Both sheets must be read together.
Water Column Pressure
27.70 inches of water column = 1 PSI
1 inch WC is approximately 1/28th of 1 PSI.
A reading of +11 WC sounds large, but it is less than 0.4 PSI. In this lesson, small WC changes matter. Do not apply PSI intuition to these readings.
Magnehelic Gauge
Port usage changes the meaning of the reading.
Lower port: connect 3/16" ID hose to measure negative pressure (vacuum).
The open port stays exposed to atmosphere.
Upper port: connect 3/16" ID hose to measure positive pressure.
Using the wrong port inverts the diagnostic meaning of the reading.
Part 2 | Figure C-1
Propane Fuel System
The propane system stores fuel as a cold, pressurized liquid and must convert it to vapor before the carburetor can use it. That liquid-to-vapor conversion is the defining feature of the propane lesson.
Propane overview
The system uses HD-5 commercial grade propane per ASTM D 1835. If the propane contains more butane than the specification allows, tank pressure at a given temperature will be lower than the chart predicts, which can contribute to fuel starvation and poor combustion.
Pipe sizing reference
| Maximum Facility Load | Required Pipe Diameter |
|---|---|
| Up to 10 kW | 3/4" |
| Up to 20 kW | 1" |
| Up to 35 kW | 1 1/4" |
| Up to 60 kW | 1 1/2" |
| Up to 125 kW | 2" |
Based on 20 feet of pipe and 5 elbows.
Component Explorer
Open only the parts you need. Critical configuration wording is preserved where precision matters.
ASCO Vapor Solenoid Valve
The ASCO solenoid is normally closed. When the engine receives a start signal, battery voltage energizes the coil and the valve opens. When the engine stops, the valve closes.
- Fails to open -> no fuel reaches carburetor -> engine will not start
- Fails open -> fuel flows when engine is off -> safety hazard; hard starting; rich running
- Used primarily on Kohler EGs; older Legacy installations may not have one
Secondary Pressure Regulator - Propane Configuration
The propane configuration is specific and must be exact:
- Mounting orientation: inverted; spring tower points down
- Regulating spring: removed
- Normal output: -0.7 to -1.7" WC vacuum at all times
If the spring is left installed, the regulator will produce positive pressure output instead of the correct slight vacuum.
Carburetor
The carburetor mixes propane vapor with combustion air. For models 200-4, 200-6, and 225, diaphragm part numbers differ by fuel type.
| Model | Propane Diaphragm |
|---|---|
| 100-64 / 125 | AV1-14-3 |
| 200-4 / 200-6 / 225 | AV1-12-2 |
| 425 | AV1-16-2 |
The carburetor test fitting cannot be used on the Model 425 carburetor. Engines equipped with the 425 (Kohler 135RZ) cannot have carburetor vacuum tested this way.
Manual Shut-Off Valve
The manual shut-off valve provides maintenance isolation and emergency shutoff. A completely closed valve produces -5 to -6" WC vacuum at the vaporizer output during cranking because the engine is pulling vacuum but no fuel is arriving.
VFF-30 Fuelock Filter and Parker PF-0532 Strainer
Legacy EGs: the VFF-30 provides filtration and vacuum-operated safety shutoff.
- Requires -2.0" WC of vacuum or greater to hold open
- Closes when engine vacuum drops to zero
- Cold or frosted housing during operation indicates a clogged element
Kohler EGs: the Parker PF-0532 is filter only; the ASCO solenoid provides shutoff.
Impco Model PE Vaporizer / Converter
The vaporizer reduces propane pressure in two stages and converts liquid propane to vapor using engine coolant heat.
- Legacy EG normal output: +4 to +11" WC
- Kohler EG normal output: +7 to +11" WC
- Kohler spring requirement: S2-42 brown spring; S2-37 is an acceptable alternative
If you see 0 to +4" WC pressure during operation on a Kohler EG, the incorrect spring is installed.
The PE has no spring-loaded pushbutton. The EB does. If an EB is installed instead of the PE, output will read -1 to -2" WC vacuum during operation instead of correct positive pressure.
If the vaporizer housing or piping shows frost or ice during operation, inadequate coolant flow is the cause. Check the coolant drain plug cavity and radiator coolant level.
Visual Walkthrough — Propane System (Figure C-1)
Step through the Figure C-1 schematic in lesson sequence. Each step highlights one instructional moment with annotated overlays on the schematic.
Model PE Diagnostic Reference
Port identification and BAL port check
- 1" NPT Outlet — vapor exits to the downstream system
- 1/8" NPT Output Pressure Test Port — at the 7 o'clock position; this is where the Magnehelic gauge connects for output pressure testing
- 1/8" NPT Internal Pressure Test Port — reads 1.5 to 3 PSI (Stage 1 internal pressure)
- 1/8" NPT Coolant Drain Plug — bottom of housing
- Vacuum Port — connects to engine intake manifold or carburetor (supplies vacuum to the VFF-30 on Legacy EGs)
- BAL Port — round perforated brass cover
If you detect propane odor coming from the BAL port, the internal diaphragm may be ruptured.
To confirm: seal the BAL port with your finger at no-load conditions. If the output pressure exceeds 11" WC with the BAL port sealed, the diaphragm is failed and must be replaced.
Pressure vs Temperature
HD-5 propane pressure / temperature reference
Propane tank pressure is directly controlled by temperature. You cannot increase tank pressure by any mechanical means — you can only change it by changing the temperature.
| Temperature (°F) | Tank Pressure | Temperature (°F) | Tank Pressure |
|---|---|---|---|
| -44 | 0 PSI | 40 | 63 PSI |
| -40 | 1.3 PSI | 50 | 77 PSI |
| -30 | 5.5 PSI | 60 | 93 PSI |
| -20 | 10.2 PSI | 70 | 109 PSI |
| -10 | 16.3 PSI | 80 | 128 PSI |
| 0 | 24 PSI | 90 | 147 PSI |
| 10 | 33 PSI | 100 | 172 PSI |
| 20 | 42 PSI | 110 | 197 PSI |
| 30 | 52 PSI | 120 | 224 PSI |
| 130 | 253 PSI |
If measured tank pressure is lower than the chart predicts for the current temperature, butane contamination or off-specification fuel is likely.
Normal Pressure Reference
| Test Point | Condition | Normal Reading |
|---|---|---|
| Vaporizer output (Legacy) | All times | +4 to +11" WC |
| Vaporizer output (Kohler) | All times | +7 to +11" WC |
| Secondary regulator output | All times | -0.7 to -1.7" WC vacuum |
| Carburetor | Cranking | -5 to -6" WC vacuum |
| Carburetor | No load to full load | -5 to -14" WC vacuum |
Pressure Reduction Sequence
- Liquid propane leaves the storage tank at temperature-dependent tank pressure.
- Filtering and shutoff are handled by the VFF-30 on Legacy EGs or Parker strainer plus ASCO on Kohler EGs.
- The Model PE vaporizer converts liquid to vapor and reduces pressure to +4 to +11" WC or +7 to +11" WC on Kohler EGs.
- The secondary regulator fine-tunes output to a slight vacuum: -0.7 to -1.7" WC.
- The carburetor responds to intake vacuum, reading -5 to -14" WC depending on condition.
Troubleshooting Concepts
Choose a symptom and compare it to the companion's diagnostic language.
Part 3 | Figure C-5
Natural Gas Fuel System
Natural gas arrives from the utility company already in vapor form. The lesson focus is how the fuel is stepped down from utility pressure and how the FAA/utility boundary changes troubleshooting responsibility.
Natural gas overview
Utility supply arrives at 5 to 30 PSI. It must be reduced twice: first by the primary regulator to roughly +11 to +14" WC, then by the secondary regulator to carburetor inlet pressure.
Natural gas has a lower heat value than propane, so the secondary regulator output is a positive pressure rather than the slight vacuum used in the propane system.
Pipe sizing reference
The pipe sizing table is identical to Figure C-1.
| Maximum Facility Load | Required Pipe Diameter |
|---|---|
| Up to 10 kW | 3/4" |
| Up to 20 kW | 1" |
| Up to 35 kW | 1 1/4" |
| Up to 60 kW | 1 1/2" |
| Up to 125 kW | 2" |
Component Explorer
The downstream layout is parallel to propane, but the pressure preparation rules are different.
ASCO Vapor Solenoid Valve
The ASCO operates identically to the propane system: normally closed, energized to open on engine start, de-energized to close on engine stop.
On Legacy EGs without the ASCO valve, there is no upstream natural gas equivalent to the VFF-30. The solenoid is the primary positive shutoff device when present.
Secondary Pressure Regulator - Natural Gas Configuration
The natural gas configuration is the opposite of propane:
- Mounting orientation: upright; spring tower points up
- Regulating spring: installed
- No load output: +5 +/-1" WC
- Full load output: +2" WC or above
The natural gas secondary regulator also requires a vent line routed to the outside of the EG building.
Carburetor
The same carburetor models and vacuum readings apply, but models 200-4, 200-6, and 225 use CV1-12-2 instead of propane diaphragm AV1-12-2.
For Kohler EGs with the Model 225 carburetor, switching from propane to natural gas requires the air/gas valve assembly change described in FAA Order JO 6980.11F section 2-13.a.
The FAA / Utility Company Boundary
The boundary sits at the exterior wall of the EG room. Utility-side incoming supply piping and meter are utility responsibility. Equipment at and downstream of the primary regulator is FAA-furnished and FAA-maintained.
If a pressure drop increasing with load traces to the supply side of the primary regulator, the issue is on the utility company side of the boundary.
Primary Pressure Regulator
The primary regulator reduces utility supply pressure from 5 to 30 PSI down to +11 to +14" WC.
- Up to 125 kW: may be mounted outside the EG building
- Over 125 kW: must be installed inside the EG room
- If mounted inside, utility supply pressure cannot exceed 5 PSI per code
Pressure that drops as load increases indicates an undersized primary regulator. Natural gas odor from the vent port indicates diaphragm or main seal failure.
Visual Walkthrough — Natural Gas System (Figure C-5)
Step through the Figure C-5 schematic in lesson sequence. Each step highlights one instructional moment with annotated overlays on the schematic.
Normal Pressure Reference
| Test Point | Condition | Normal Reading |
|---|---|---|
| Primary regulator output | All times | +11 to +14" WC |
| Secondary regulator input | All times | +11 to +14" WC |
| Secondary regulator output | No load | +5 +/-1" WC |
| Secondary regulator output | Full load | +2" WC or above |
| Carburetor | Cranking | -5 to -6" WC vacuum |
| Carburetor | No load to full load | -5 to -14" WC vacuum |
Pressure Reduction Sequence
- Utility gas arrives at 5 to 30 PSI.
- The primary regulator reduces utility pressure to +11 to +14" WC.
- The secondary regulator fine-tunes output to +5 +/-1" WC at no load and +2" WC or above at full load.
- The carburetor vacuum readings remain -5 to -14" WC depending on condition.
- The FAA/utility boundary determines who owns the upstream pressure problem.
Troubleshooting Concepts
Use the companion's test-point logic to separate upstream supply issues from regulator issues.
Part 4
Compare Systems
Both systems feed the same carburetor types using the same downstream layout. The important differences are upstream preparation, pressure staging, ownership boundary, and the opposite secondary regulator setup.
Propane
Liquid at source
Requires a vaporizer and coolant-supported liquid-to-vapor conversion before entering the building in the Figure C-1 lesson configuration.
Natural Gas
Gas at source
Arrives from the utility company already in vapor form and must be stepped down from 5 to 30 PSI using a primary regulator.
Critical difference
Same regulator, opposite setup
Propane output is a slight vacuum. Natural gas output is positive pressure. That difference drives both configuration and diagnosis.
Focus the Comparison
Use the buttons to spotlight one type of difference at a time.
| Attribute | Propane System (Figure C-1) | Natural Gas System (Figure C-5) |
|---|---|---|
| Fuel source | On-site propane storage tank (FAA-owned) | Utility company piped supply |
| Fuel state at source | Liquid, high pressure | Gas, already at pressure |
| Pressure at source | Temperature-dependent | 5-30 PSI |
| Pressure preparation | Two-stage vaporizer: liquid to vapor; tank pressure to 4-11" WC or 7-11" WC on Kohler EGs | Primary regulator: 5-30 PSI to 11-14" WC |
| Upstream safety shutoff | VFF-30 on Legacy EGs or ASCO on Kohler EGs | ASCO solenoid only |
| Utility / FAA boundary | None; FAA owns everything | At exterior wall of EG room |
| Downstream regulator stages | Secondary regulator only | Primary plus secondary regulators |
Secondary Regulator Configuration
| Configuration Attribute | Propane | Natural Gas |
|---|---|---|
| Mounting orientation | Inverted; spring tower down | Upright; spring tower up |
| Regulating spring | Removed | Installed |
| Normal output at no load | -0.7 to -1.7" WC vacuum | +5 +/-1" WC positive pressure |
| Normal output at full load | -0.7 to -1.7" WC vacuum | +2" WC or above |
| Vent line | No | Yes; routed outside the EG building |
Normal Pressure Comparison
| Test Point | Propane (Legacy) | Propane (Kohler) | Natural Gas |
|---|---|---|---|
| Upstream output | +4 to +11" WC | +7 to +11" WC | +11 to +14" WC |
| Secondary regulator input | +4 to +11" WC | +7 to +11" WC | +11 to +14" WC |
| Secondary regulator output (no load) | -0.7 to -1.7" WC vacuum | -0.7 to -1.7" WC vacuum | +5 +/-1" WC |
| Secondary regulator output (full load) | -0.7 to -1.7" WC vacuum | -0.7 to -1.7" WC vacuum | +2" WC or above |
| Carburetor (cranking) | -5 to -6" WC vacuum | -5 to -6" WC vacuum | -5 to -6" WC vacuum |
| Carburetor (no load to full load) | -5 to -14" WC vacuum | -5 to -14" WC vacuum | -5 to -14" WC vacuum |
Applied Practice
Troubleshooting
Treat readings as diagnostic indicators. Start with the exact test point and condition, compare it to the normal range, then use the abnormal signature that matches.
Pressure reading interpretation
Always identify the test point first, then the operating condition, then the expected WC range.
Component failure interpretation
A positive propane secondary regulator output, an EB vaporizer signature, or a high natural gas output each point to different hardware problems.
Guided diagnostic thinking
Work from the most accessible upstream restriction toward the less accessible regulating components unless the reading points directly at a misconfiguration.
Scenario Practice
Choose a scenario to reveal the likely interpretation and the next check named in the companion.
Parts 6 and 7
Final Review
Use the key takeaways for fast recall, then open the questions for exam-style practice. Answers are kept concise and tied to the lesson language.
Water column is the unit. PSI intuition does not apply.
The secondary regulator is installed in opposite orientations for each fuel type.
Normal secondary regulator output is on opposite sides of zero.
In the Figure C-1 lesson configuration, the vaporizer is outside the EG building.
Two vaporizer models exist. The PE is correct; the EB is not.
The Kohler EG requires the correct brown spring.
The VFF-30 and ASCO solenoid have different roles depending on EG type.
The FAA / utility boundary exists only in the natural gas system.
The 125 kW threshold governs primary regulator placement.
The carburetor test fitting has one exception: the Model 425.
Models 200-4, 200-6, and 225 use different diaphragm part numbers by fuel type.
Pipe sizing is identical for both systems.
Foundation and Propane
1. WC to PSI conversion
27.70 inches WC = 1 PSI. The companion states this in Note 1 on all four schematic sheets.
2. Gauge connected to lower port but reading positive
The lower port is for vacuum. The setup is inverted for a positive pressure reading. Use the upper port when measuring positive pressure.
3. What is 5" WC in PSI?
Approximately 0.18 PSI. The distinction matters because these are very low pressures and small WC changes are diagnostically meaningful.
4. Kohler propane fuel path
Storage tank -> manual shut-off valve -> Parker PF-0532 FPT strainer -> Impco Model PE vaporizer -> ASCO solenoid -> secondary regulator -> carburetor -> engine.
5. VFF-30 functions and opening requirement
It provides filtration and vacuum-operated safety shutoff. It requires -2.0" WC of vacuum or greater to hold open.
6. Frost on VFF-30 housing
The element is clogged. Replace it.
7. Why the vaporizer uses engine coolant
Liquid propane absorbs heat during pressure drop. Coolant supplies thermal energy so the housing or piping does not frost or freeze and restrict vaporization.
8. Kohler vaporizer output reads 0 to +4" WC
The incorrect spring is installed. Replace it with the brown S2-42 spring or acceptable S2-37 alternative.
9. Vaporizer with a spring-loaded pushbutton
That identifies an EB model, not the correct PE. Convert it using conversion kit NSN 0000-00-012-1703.
10. Figure C-1 vaporizer location rule
In the lesson configuration it is outside the EG building because liquid propane is not piped inside; fuel is converted to vapor before entering the building.
Natural Gas and Comparison
11. Normal primary regulator output
+11 to +14" WC at all times.
12. 175 kW natural gas unit regulator location
The primary regulator must be inside the EG room. If it is inside, utility supply pressure cannot exceed 5 PSI per code.
13. Why natural gas uses two pressure reductions
Utility gas arrives at 5 to 30 PSI, so it must first be reduced by the primary regulator to +11 to +14" WC and then fine-tuned by the secondary regulator. Propane uses the vaporizer for upstream preparation and the secondary regulator for final control.
14. Kohler natural gas fuel path
Utility supply -> utility equipment -> FAA / utility boundary -> manual shut-off valve -> primary regulator -> ASCO solenoid -> secondary regulator -> carburetor -> engine.
15. FAA / utility boundary meaning
It is at the exterior wall of the EG room. Utility-side incoming supply and meter are utility responsibility; downstream of the primary regulator is FAA responsibility.
16. Secondary regulator setup by fuel type
Propane: inverted, spring removed, -0.7 to -1.7" WC vacuum. Natural gas: upright, spring installed, +5 +/-1" WC at no load.
17. Propane secondary regulator reads +4" WC
The regulating spring has likely been left installed.
18. Natural gas secondary regulator reads +8" WC at full load
Abnormal. +6" WC or above during operation indicates regulator binding or a clogged vent line.
19. Propane vaporizer output reads -1" WC during operation
The system likely has an EB vaporizer installed. Convert it to PE specification with NSN 0000-00-012-1703.
20. Natural gas input shows -5" WC during cranking
No fuel reaching regulator — check ASCO solenoid, check primary regulator, check manual shut-off valve.
21. Carburetor models with different diaphragm part numbers
Models 200-4, 200-6, and 225. Propane uses AV1-12-2. Natural gas uses CV1-12-2.
22. Pipe diameter for a 60 kW engine generator
1 1/2". The answer does not change between propane and natural gas.
Diagnostics and Procedures
23. BAL port diaphragm test
At no load, seal the BAL port with your finger. If output pressure exceeds 11" WC with the port sealed, the diaphragm is failed.
24. Kohler 100RZ reads -16" WC at load
That is normal for the Kohler 100RZ with the Model 225 carburetor. The companion states that vacuum at load will exceed -14" WC for that specific unit.
25. Cannot install the carburetor test fitting
The engine is using a Model 425 carburetor, associated in the companion with the Kohler 135RZ. Carburetor vacuum cannot be tested with the kit fitting on that unit.
26. Propane secondary regulator output reads -3" WC during cranking
That suggests the regulator is binding mechanically or the vent line is clogged.
Lesson Summary
The companion's core job is to help students recognize which readings are normal, which signatures point to misconfiguration, and which differences between propane and natural gas matter most in field diagnosis. Use the comparison tables and troubleshooting scenarios as the primary bridge from study to hands-on work.