Natural Gas Engines

Hannu Jääskeläinen

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Abstract: Natural gas engines can range from small light-duty engines to low speed two-stoke marine engines of over 60 MW. The dominant engine cycle could be either Otto or Diesel, using several different techniques of mixture preparation and ignition. Most commercial and development natural gas engines can be categorized into four types of technology: (1) stoichiometric Otto cycle engines; (2) lean burn, Otto cycle engines; (3) dual fuel mixed cycle (combination of Otto and Diesel) engines, and (4) diesel cycle natural gas engines. These technologies exhibit differences in thermal efficiency, performance, and aftertreatment requirements.

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. 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]:

[schematic] [schematic] [schematic]
Figure 1. Three categories of natural gas engines

(Source: Wärtsilä)

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 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].

Table 1
Comparison of different combustion systems for natural gas engines
Stoichiometric Otto CycleLean Burn Otto CycleDual Fuel Mixed CycleDiesel Cycle
State of air/fuel mixturePremixedNo premixing
Overall AFRStoichiometricLean
Dominant engine cycleOttoOtto/DieselDiesel
TechnologyIgnition options
  • Spark plug, open chamber
  • Spark plug, open chamber
  • Spark plug, pre-chamber (passive or active)
  • Diesel micro-pilot, open chamber
  • Diesel micro-pilot, pre-chamber
  • Glow plug, pre-chamber (limited application)
  • Diesel pilot, open chamber
  • Diesel pilot, open chamber
  • Glow plug, open chamber (experimental)
  • Spark plug, pre-chamber (experimental)
Control of engine-out emissions
  • NOx: EGR, Ignition timing
  • CH4: combustion chamber crevice volumes, scavenging flow, closed crankcase ventilation (CCV)
  • PM: oil consumption
  • NOx: AFR, ignition timing
  • CH4: combustion chamber crevice volumes, scavenging flow, CCV, bulk combustion losses
  • PM: oil consumption
  • NOx: AFR, diesel pilot qty., ignition timing
  • CH4: combustion chamber crevice volumes, scavenging flow, CCV, bulk combustion losses
  • PM: diesel pilot qty., oil consumption
  • NOx: EGR, injection timing
  • PM: similar to diesel
Aftertreatment system (ATS) options
  • TWC for NOx, CH4, CO
  • PM: no ATS needed up to US 2010 and Euro VI-D
  • NOx: Urea SCR
  • CH4: MOC in limited applications
  • NOx: Urea SCR
  • CH4: MOC in limited applications
  • NOx: Urea SCR
  • CH4: not typically needed
  • PM: DPF (active regeneration requires DOC + diesel fuel)
Primary applications
  • Light-, medium- and heavy-duty
  • Stationary < ~1 MW
  • Stationary and marine
  • Rail and large non-road, diesel retrofits
  • Heavy-duty, stationary and marine
Efficiency, BTE, without WHR
  • <40%, commercial engines; ~45% BTE potential
  • <50%, commercial engines
  • <47%, commercial engines
  • Heavy-duty: <46%; Efficiency potential similar to diesel, ~50%
  • Low-speed marine: <48%, commercial engines
Advantages
  • 100% diesel substitution
  • Low emissions of NOx and CH4
  • Simple passive ATS
  • Works with CNG or LNG
  • High efficiency
  • Can avoid use of spark plugs
  • Diesel-only operation possible (dual fuel only)
  • Works with CNG or LNG
  • 100% diesel substation (except diesel micro-pilot)
  • Up to >99% diesel substitution with diesel micro-pilot
  • High efficiency
  • No spark plugs
  • Diesel-only operation possible
  • Retrofit of existing diesel engines possible
  • Works with CNG or LNG
  • High power density
  • Knock resistant
  • High efficiency
  • Can avoid use of spark plugs
  • Up to 95% diesel substitution
  • Low CH4 emissions
  • Robust to changes in fuel gas composition
Challenges
  • Spark plug life
  • Lower power density relative to diesel
  • Low efficiency relative to diesel
  • High load operation can be knock limited
  • Spark plug life (spark ignited only)
  • Unburned CH4 emissions
  • High load operation on NG can be knock limited
  • Diesel substitution limited to ~50-85%
  • Misfire at light-load with NG
  • Unburned CH4 emissions
  • High load operation on NG can be knock limited
  • Diesel-only operation not possible
  • LNG only for mobile applications. CNG requires high power compressor with large footprint
  • High cost and complexity
  • PM and NOx require full diesel ATS (heavy-duty)

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