Natural Gas Engines

Hannu Jääskeläinen

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• Introduction
• Premixed Engines
• Non-premixed Engines

Introduction

The low cost of natural gas relative to diesel and gasoline combined with various emissions related regulatory measures continues to created significant interest in natural gas as an alternative fuel for internal combustion engines. Engine makes have responded by supplying new, purpose built natural gas engines in sizes ranging from small light-duty engines of a few kW to low speed two-stoke marine engines of over 60 MW. In 2019, WinGD claimed their 12X92DF dual fuel engine to be the most powerful Otto cycle engine at 63,840 kW [4829]. OEMs and aftermarket suppliers also provide conversion kits that allow existing diesel and gasoline engines to be converted to operate on natural gas.

Natural gas engines can be categorized based on numerous parameters including: mixture preparation (premixed or non-premixed), ignition (spark ignition or diesel pilot) and the dominant engine cycle (Otto or Diesel). One common categorization is, Figure 1 [4247]:

• Premixed charge, spark ignition, natural gas only
• Premixed charge, diesel pilot ignition, natural gas/diesel dual fuel
• High pressure direct injection of natural gas, diesel pilot ignition, natural gas/diesel dual fuel

While the above grouping adequately covers commercial engine sizes up to about 2.5 L/cylinder, when larger engines are also considered, it creates some challenges in presenting common concepts between some of the different approaches. More specifically, lean burn dual fuel engines ignited by a small (<~5% fuel energy) diesel micro-pilot share more in common with lean burn SI engines than they do with dual fuel engines using a much larger diesel pilot (>~15% fuel energy). It also does not cover some concepts in the development stage. The following categorization is more general and reflects common concepts between the different approaches:

• Stoichiometric Otto cycle engines
• Lean burn Otto cycle engines
• Dual fuel mixed cycle (combination of Otto and Diesel) engines
• Diesel cycle natural gas engines

Stoichiometric Otto cycle engines use a premixed “near-stoichiometric” air-fuel mixture and are ignited with a spark plug. An important motivation for using stoichiometric engines is the fact that they can utilize a three-way catalyst (TWC), sometimes also referred to as a non-selective catalytic reduction (NSCR) catalyst, to reduce NOx and oxidize CO and hydrocarbons in the exhaust. It should be noted that the peak conversion efficiency for NOx, CO and HC in a TWC with natural gas is just rich of stoichiometry and natural gas engines burning a “stoichiometric” air-fuel mixture are typically calibrated to operate slightly rich. This is reflected in the terminology used for stationary natural gas engines for which natural gas engines using a near-stoichiometric mixture are sometimes referred to as “rich-burn” engines.

Lean-burn Otto cycle engines use a lean premixed air-fuel mixture with several options for ignition. A spark plug or a diesel micro-pilot are the two most common options. Glow plugs have also seen limited commercial application. One important benefit of lean burn Otto cycle engines is their high brake thermal efficiency (BTE) that can reach 50% in many cases. If aftertreatment is required on lean burn engines, urea SCR is an option for NOx control. Methane oxidation catalysts require high exhaust temperature to be effective and are only useful in some stationary applications.

Dual fuel mixed cycle engines use a lean premixed air-fuel mixture ignited by a substantial diesel pilot representing more than ~15% of the total fuel energy. They are referred to here as mixed cycle engines because the diesel pilot contributes significantly to the total heat release during combustion of the premixed natural gas/air charge. An important benefit of this approach is that existing diesel engines (either in-use engines or existing diesel engine platforms from an engine maker) can be relatively easily converted to use natural gas—a popular consideration when the price differential between diesel and natural gas is large.

Diesel cycle natural gas engines do not premix the natural gas with air. Instead, natural gas is injected directly into the combustion chamber at high pressure in much the same way that is done in a diesel engine. However, unlike diesel engines, an ignition source is required. The primary means of igniting the natural gas jets is to ignite a small diesel pilot just prior to the injection of the gas. This approach is sometimes referred to as high pressure direct injection (HPDI) or gas-diesel. Ignition via a glow plug or a pre-chamber spark plug is also being researched. An important benefit of this approach is that a higher power density is achievable and a higher compression ratio can be used compared to premixed approaches.

Table 1 summarizes these approaches with further details provided below. Other summaries similar to Table 1 are available but primarily focus only on heavy-duty applications [3568][4323].

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