What Does a “Diesel-Gas Generator” Mean?
A “diesel-gas generator” most commonly refers to a dual-fuel generator set—an industrial engine designed to burn a mixture of diesel fuel and a gaseous fuel (usually natural gas) simultaneously. Contrary to a common misunderstanding, these are not generators that can be switched to run exclusively on either diesel or gas. Instead, a dual-fuel generator initially starts on 100% diesel fuel and, as load increases, progressively blends in natural gas, allowing the engine to run on a combination of both fuels Bi Fuel Engines. The diesel pilot remains essential for ignition, eliminating the need for a spark plug.
This technology offers a compromise: the operational flexibility and lower fuel cost of natural gas with the load-acceptance and starting reliability of a diesel compression-ignition engine. For facility operators and EPCs, understanding the blending process, gas-quality requirements, and the effects on generator rating is critical before selecting this system for standby, prime, or continuous power.
How Dual-Fuel Blending Works
The blending process is not a static 50/50 split; it is a dynamic ratio controlled by the engine management system.
- Diesel-Only Start & Low-Load: The engine starts and idles on 100% diesel to ensure reliable ignition and stable combustion.
- Gas Introduction: As the engine load increases above a preset threshold (typically 30–40% of rated load), a control system meters gaseous fuel into the intake air stream.
- Substitution Dynamics: At higher loads, natural gas can replace a significant portion of the diesel. The maximum “substitution rate” varies by engine design, load, and gas quality but often reaches 50–70%. The diesel injection is reduced to just a small “pilot” quantity sufficient to ignite the lean gas-air mixture.
The system continuously monitors engine parameters (knock, exhaust temperature, speed) to adjust the diesel-gas ratio, ensuring safe operation without detonation.
Gas Types and Fuel Quality Implications
The performance of a diesel gas generator is highly sensitive to the type and quality of the gaseous fuel. The engine’s knock resistance and energy content are the primary concerns.
- Pipeline Natural Gas: This is the most consistent fuel source. The methane number (MN)—an analog to gasoline’s octane rating—typically ranges high enough to allow high substitution rates without engine knock. A high MN resists uncontrolled auto-ignition.
- Associated Petroleum Gas (APG) / Flare Gas: In oil fields, this fuel is abundant but often contains heavier hydrocarbons like ethane, propane, and butane. These components lower the methane number significantly, increasing the risk of destructive engine knock. Without aggressive derating, a rich APG source can damage pistons and cylinder heads.
- Biogas (Anaerobic Digester Gas): Often sourced from landfills or agricultural waste, biogas contains methane, CO₂, and corrosive contaminants like hydrogen sulfide (H₂S). The low energy density (low BTU value) due to CO₂ reduces engine power output. Moreover, H₂S requires specialized fuel treatment to prevent corrosion of aftercoolers and exhaust components.
Key point: An engine’s allowable substitution rate and maximum power output will be lower when operating on low-MN or low-BTU gas compared to clean pipeline gas. A site-specific gas analysis is a mandatory engineering input.
Ratings and Power Derating Considerations
A generator’s nameplate kW or kVA rating is established under specific reference conditions, typically for 100% diesel operation. When you switch to dual-fuel mode, that rating cannot be taken for granted.
- Steady-State Power: The introduction of natural gas reduces the energy density of the intake charge compared to pure air. A naturally aspirated dual-fuel engine will experience a power loss. Turbocharged engines can sometimes compensate by increasing boost pressure, but only to a design limit. Expect a 10–20% reduction in maximum continuous power compared to diesel-only rating, depending on gas quality. Suppliers provide specific dual-fuel derating curves.
- Load Acceptance (Transient Response): This is often the most constrained parameter. Gas-air mixing creates a less instantly combustible charge than a direct diesel injection spray. A dual-fuel generator that meets the ISO 8528 G2 voltage and frequency recovery class on diesel may fall to G3 performance (greater deviation, slower recovery) in dual-fuel mode. If the generator is expected to accept large block loads (e.g., starting a large motor), the project must specify and verify the transient performance in the intended dual-fuel operating mode. This is not a typology covered by standard diesel-only datasheets.
Commercial and Regulatory Drivers for Dual-Fuel
The decision to adopt a dual-fuel system is usually driven by one of three factors:
- Fuel Cost Reduction: In regions where natural gas is significantly cheaper than diesel per unit of energy, replacing 60% of diesel consumption can drastically lower the levelized cost of energy for prime-power applications like remote mines or off-grid industrial plants.
- Extended Run-Time Autonomy: A fixed volume of diesel fuel storage can last much longer when supplemented by a continuous gas supply. This reduces refueling logistics and can be critical for sites that are inaccessible for long periods.
- Emissions Compliance: Burning natural gas produces less particulate matter (PM) and sulfur oxides (SOx) than diesel. In settings like non-emergency power generation, dual-fuel operation can help meet stricter emissions limits. For instance, non-emergency generators installed after January 1, 2009, must meet emission limits of 1.5 lbs NOx per MWh, 0.07 lbs PM per MWh, and 2.0 lbs CO per MWh Chapter 148: Emissions from Smaller-Scale Electric …. A well-managed dual-fuel system with exhaust aftertreatment can be part of the compliance strategy.
Application Fit: When to Choose a Diesel-Gas Generator
A dual-fuel system is not a universally superior drop-in replacement. Evaluate the fit against your operational profile:
Suitable Applications:
- Prime or Continuous Power: High-hour facilities (factories, mines, remote communities) where the economic payback from fuel substitution is substantial.
- Stable Base Loads: Applications where the load profile rarely drops below 30%, keeping the engine in the blending zone.
- Sites with Uninterrupted Gas Supply: A reliable supply with consistent, high methane-number gas is required.
Negative-Fit Conditions:
- Standby/Emergency Duty: Emergency generators are limited to maintenance and testing hours Chapter 148: Emissions from Smaller-Scale Electric …. A complex dual-fuel system adds capital cost and maintenance points with minimal opportunity to recoup fuel savings.
- Frequent Load Dumps or Idling: Constant cycling between diesel-only and blended modes reduces savings and adds control complexity.
- Unstable or Low-MN Gas Sources: The risk of engine damage from knock undermines reliability.
Vendor Selection Method and RFQ Preparation
Selecting a diesel gas generator requires more due diligence than a standard diesel set. Your RFQ package must address the blended operating mode explicitly.
1. Define the Duty Cycle in Blended Mode
Specify the number of hours per year in diesel-only and in dual-fuel mode at specific average loads. A unit may require a specific prime rating applicable to dual-fuel use for non-emergency generation.
2. Demand a Site-Specific Performance Curve
Do not accept a standard diesel-only datasheet. Require the manufacturer to provide:
- A power-speed curve with a derating factor for your specific gas’s methane number and lower heating value (BTU).
- Transient voltage and frequency response curves for block loads (e.g., 0–50%, 0–70% step loads) in dual-fuel mode.
- A guaranteed maximum allowable substitution rate at 100% load without knock.
3. Validate Fuel System Engineering
Request a materials compatibility statement for gas at your site pressure, temperature, and contaminant level. Confirm the supplied gas train includes filtration, pressure regulation, and a fast-acting shut-off valve tied to the engine’s safety system.
Application Constraints and Verification
Before contracting, verify these critical boundary conditions with your supplier:
- Knock Detection and Protection: The engine management system must include cylinder-specific knock sensors with an automatic strategy to reduce substitution rate or, in severe cases, trip to full diesel if knock is detected.
- Maintenance Impact: Gas combustion is hotter. Valves, pistons, and cylinder heads may require different materials or a shortened maintenance interval compared to a pure diesel engine of the same block.
- Start Reliability: In prime power applications with an unreliable gas grid, the control system must allow a seamless transition to 100% diesel operation without a power interruption to the load. The generator rating on 100% diesel must be sufficient to carry the full site load.
Buyer’s Verification Checklist
Use this checklist when evaluating a diesel-gas generator proposal:
- Gas Analysis Approved: Has the supplier issued a formal derating and substitution commitment based on our site-specific gas analysis (methane number, LHV, H₂S content)?
- Blended-Mode Rating: Is the prime or continuous kW/kVA rating clearly stated for the specific dual-fuel blend, not just a diesel-only rating?
- Transient Performance: Have voltage/frequency dip and recovery times been validated to project limits for worst-case motor starting in dual-fuel mode?
- Emission Compliance Evidence: Does the engine’s EPA or local emission certification explicitly cover dual-fuel operation at the claimed substitution rate?
- Fuel System Compatibility: Are all gas components (valves, regulators) rated for the maximum site gas pressure and compatible with contaminant levels?
- Protection Logic: Is the knock-mitigation strategy and automatic diesel-only fallback logic defined and proven?
- Service Plan: Does the supplier’s warranty and maintenance contract cover valve recession, cylinder head condition, and top-end components under dual-fuel operation?
A diesel-gas dual-fuel generator is a powerful tool for operational cost reduction, but its success hinges on a precise match between the engine’s combustion capabilities and the specific, gravity, and purity of the site’s gaseous fuel. Treat the blended operating mode as a distinct engineering configuration, not an option checkbox, and insist the supplier verifies all ratings and performance guarantees against your exact fuel and load profile.
