CNG, LPG and hydrogen as engine fuels

How CNG, LPG and hydrogen are stored and burned in engines, their heating values, air-fuel ratios, octane and flame properties, the power penalty and emission changes they bring, and the safety points that follow from their physical properties.

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Why it matters

CNG runs a large share of India's autorickshaws, taxis, city buses and an increasing number of factory-fitted cars; LPG is used in autos and converted cars; and hydrogen engines and fuel cells are being trialled for trucks and buses. An automotive engineer must know how each gas is stored, how it changes power, knock behaviour and emissions, and what has to change in the engine and fuel system — including the safety reasoning behind cylinder pressures and leak handling.

Key ideas

Compressed natural gas (CNG).

  • Composition: mostly methane (CH₄), typically above about 85–90%, with small amounts of ethane, propane, CO₂ and N₂ depending on the source.
  • Storage: as a gas in steel or composite cylinders at about 200 bar (20 MPa). Even at this pressure the volumetric energy density is low, so cylinders are bulky and range is limited.
  • Properties: lower heating value about 50 MJ/kg; density about 0.72 kg/m³ at 0 °C and 1 atm, so about 36 MJ per standard m³; stoichiometric air–fuel ratio about 17.2 by mass; very high knock resistance (octane number around 120); high autoignition temperature; slower flame speed than petrol, so more spark advance is needed.
  • In engines: used in SI engines — bi-fuel petrol/CNG cars with a pressure regulator (reducer) and gas injectors, or dedicated CNG engines with higher compression ratio. A converted petrol engine typically loses roughly 10% of its power because the gas displaces air and the stoichiometric mixture carries less energy per unit volume (Example 2). Diesel engines can be converted to CNG spark ignition or run dual-fuel with a pilot diesel injection.
  • Emissions: almost no particulate matter; lower CO; about 20% less CO₂ per unit energy than petrol because of its high hydrogen-to-carbon ratio; unburnt HC is mostly methane, a strong greenhouse gas that is hard to oxidise in a catalyst. NOx still needs a three-way catalyst on a stoichiometric engine.

Liquefied petroleum gas (LPG).

  • Composition: propane (C₃H₈) and butane (C₄H₁₀) in proportions that vary by season and supplier; an odorant is added to detect leaks.
  • Storage: as a liquid under its own vapour pressure, typically several bar (roughly 5–10 bar at ambient temperature, depending on composition and temperature). Tanks are filled to only about 80% to leave room for liquid expansion.
  • Properties: lower heating value about 46 MJ/kg; liquid density about 0.5–0.58 kg/L, so roughly 25 MJ/L against about 32 MJ/L for petrol; stoichiometric AFR about 15.5; high octane (propane is well above 100).
  • In engines: SI engines with a vaporiser-regulator and gas mixer or vapour/liquid injection. Range per litre of tank is lower than on petrol.
  • Safety: LPG vapour is heavier than air, so leaks collect in low places — a key difference from CNG and hydrogen, which rise and disperse.

Hydrogen (H₂).

  • Properties: lower heating value 120 MJ/kg (the highest of any fuel per kg) but only about 10.8 MJ per standard m³; stoichiometric AFR about 34 by mass; wide flammability range (about 4–75% by volume in air); very low minimum ignition energy; laminar flame speed several times that of petrol; small quenching distance; high autoignition temperature.
  • Storage: compressed gas at 350 or 700 bar in composite tanks, or liquid at about −253 °C (20 K) with boil-off losses. Production route decides its environmental value: hydrogen from natural gas reforming without carbon capture still emits CO₂ upstream; electrolysis with renewable electricity gives "green" hydrogen.
  • In IC engines: burns to water vapour, so there is no CO, HC, CO₂ or soot from the fuel (only traces from lubricating oil). NOx is the one pollutant; it is controlled by running very lean (the wide flammability limit allows λ well above 2) or with EGR and after-treatment. Problems are pre-ignition, backfire into the intake and knock-like abnormal combustion caused by the low ignition energy and fast flame, plus loss of power with port injection because hydrogen occupies about 30% of the stoichiometric mixture volume. Direct injection after inlet valve closure overcomes backfire and the power loss. Hydrogen is a poor CI fuel on its own because of its high autoignition temperature; dual-fuel operation uses a diesel pilot.
  • Fuel cells: a fuel cell converts hydrogen electrochemically to electricity, with higher efficiency than an IC engine at part load; it is an alternative to burning hydrogen, not an engine.

Formulas

E = m × LHV or E = V × LHV_v

  • E = energy content (MJ); m = mass (kg) with LHV in MJ/kg; V = volume (m³ at standard conditions, or L of liquid) with LHV_v in MJ/m³ or MJ/L. Always state the reference state for a gas volume.

AFR_st = [(x + y/4) × 4.76 × 28.97] / M_fuel

  • For a hydrocarbon CₓHᵧ (H₂ with x = 0, y = 2): AFR_st in kg air/kg fuel; M_fuel = molar mass (g/mol).

x_fuel = 1 / (1 + 4.76 × (x + y/4))

  • Mole (volume) fraction of fuel in a stoichiometric gaseous fuel–air mixture.

E_mix = x_fuel × ρ_fuel × LHV

  • E_mix = energy per m³ of stoichiometric mixture (MJ/m³); ρ_fuel = fuel density at standard conditions (kg/m³) = M / 22.414 at 0 °C and 101.325 kPa. Power of a gas-fuelled, port-injected engine is roughly proportional to E_mix.

CO₂ per MJ = (44 x / M_fuel) / LHV

  • kg CO₂ per kg fuel divided by LHV (MJ/kg), giving kg CO₂/MJ.

Composition, LHV and density of natural gas and LPG vary by supply; use the data-book or given values.

Worked examples

Example 1 (standard): range-equivalent of a CNG cylinder. Given: a car's CNG cylinders hold 12 kg of gas with LHV 50 MJ/kg; petrol has 32 MJ/L. Assume equal engine efficiency. Find the energy stored and the petrol-equivalent volume.

  1. E = m × LHV = 12 × 50 = 600 MJ.
  2. Petrol equivalent: V = 600 / 32 = 18.75 L. Answer: 600 MJ, equal to about 18.8 L of petrol — which is why CNG cars carry a petrol tank as back-up.

Example 2 (GATE level): power loss on converting a petrol engine to CNG. Given: methane CH₄ (M = 16.04 g/mol, LHV = 50.0 MJ/kg) and petrol as iso-octane vapour C₈H₁₈ (M = 114.23 g/mol, LHV = 44.4 MJ/kg); stoichiometric, premixed, port injection; standard molar volume 22.414 m³/kmol. Estimate the power of the CNG engine relative to petrol.

  1. Methane: x_fuel = 1 / (1 + 4.76 × 2) = 0.0951; ρ = 16.04 / 22.414 = 0.716 kg/m³.
  2. E_mix,CH₄ = 0.0951 × 0.716 × 50.0 = 3.40 MJ/m³.
  3. Iso-octane: x_fuel = 1 / (1 + 4.76 × 12.5) = 0.01653; ρ = 114.23 / 22.414 = 5.096 kg/m³.
  4. E_mix,petrol = 0.01653 × 5.096 × 44.4 = 3.74 MJ/m³.
  5. Ratio: 3.40 / 3.74 = 0.909. Answer: about 91% of petrol power (≈ 9% loss) before allowing for the higher compression ratio a dedicated CNG engine can use. The same method gives about 3.19 MJ/m³ for hydrogen (x_fuel = 0.296), roughly 85% of petrol.

Example 3: CO₂ advantage of CNG.

  1. Methane: 44 × 1 / 16.04 = 2.743 kg CO₂/kg; per MJ: 2.743 / 50.0 = 0.0549 kg/MJ = 54.9 g/MJ.
  2. Iso-octane: 44 × 8 / 114.23 = 3.082 kg CO₂/kg; per MJ: 3.082 / 44.4 = 69.4 g/MJ.
  3. Reduction: 1 − 54.9 / 69.4 = 0.21. Answer: about 21% less CO₂ per MJ of fuel energy.

Common mistakes

  • Quoting CNG energy density as about 9.5 MJ/m³ — that is close to its energy in kWh per m³ (≈ 10 kWh/m³). The lower heating value is about 36 MJ per standard m³.
  • Comparing hydrogen per kg only; per litre of storage it is far poorer than liquid fuels.
  • Calling hydrogen "zero-emission" in an IC engine: NOx is still produced, and upstream CO₂ depends on how it was made.
  • Assuming LPG is mainly methane (that is CNG) or that it is stored at CNG-like pressures.
  • Forgetting the volumetric (power) penalty of gaseous fuels displacing air in a port-injected engine.
  • Treating a gas volume without stating temperature and pressure.
  • Thinking high octane fuels need less spark advance — slow-burning methane actually needs more.

For GATE ME

Questions are mostly conceptual: matching fuel to storage method and main constituent, octane and flame-speed effects, emission advantages and limits, and safety (density relative to air, flammability range). Numericals cover energy content, petrol-equivalent volume, stoichiometric AFR, mixture energy density and CO₂ per unit energy. Practise the stoichiometric balance and density from molar volume.

Quick check

  1. At about what pressure is CNG stored in vehicle cylinders?
  2. Why does an LPG leak pose a greater pooling hazard than a CNG leak?
  3. What is the stoichiometric AFR of hydrogen by mass (approximately)?
  4. Name the one significant pollutant from a hydrogen IC engine and one way to control it.
  5. Why does a port-injected gas engine lose power compared with petrol?

Answers: 1. About 200 bar (20 MPa). 2. LPG vapour is heavier than air; methane is lighter and disperses. 3. About 34. 4. NOx; very lean operation (or EGR/after-treatment). 5. The gaseous fuel displaces intake air, lowering the mixture energy per unit volume.

Interview questions

All Automotive Engines, Emissions and Alternate Fuels interview questions

Try answering each one aloud before you open it.

  1. 1.What is Compressed Natural Gas (CNG) and how is it used as a fuel in automotive engines?Concept

    CNG is natural gas, mostly methane, compressed to about 200 bar in steel or composite cylinders. It is burned in spark-ignition engines, either bi-fuel petrol/CNG engines with a pressure regulator and gas injectors or dedicated engines with a higher compression ratio to exploit its octane number of about 120. It gives almost no particulate, low CO and about 20% less CO₂ per unit energy than petrol, but the bulky cylinders limit range and a converted petrol engine loses roughly 10% of its power because the gas displaces intake air.

  2. 2.Explain the advantages and disadvantages of using Liquefied Petroleum Gas (LPG) as a fuel in vehicles.Concept

    LPG, a mixture of propane and butane, is used as an automotive fuel due to its clean-burning properties. Advantages include lower emissions of CO2 and other pollutants compared to gasoline, and it is often cheaper. Disadvantages include a lower energy density than gasoline, which can result in reduced range, and the need for specialized storage tanks due to its pressurized state.

  3. 3.Describe how hydrogen can be used as a fuel in automotive engines.Concept

    Hydrogen can be used in two main ways: in internal combustion engines modified to burn hydrogen, or in fuel cells that convert hydrogen into electricity to power electric motors. Hydrogen combustion produces only water vapor as a byproduct, making it a clean fuel. However, challenges include storage, distribution, and the energy-intensive process of hydrogen production.

  4. 4.Why is CNG considered a cleaner alternative to gasoline and diesel?Application

    Methane is a single, light molecule that mixes completely with air as a gas, so combustion produces almost no soot and little CO, and the fuel contains no sulphur or aromatics. Its high hydrogen-to-carbon ratio gives about 20% less CO₂ per unit of energy than petrol and somewhat less than diesel. NOx is not automatically low — a stoichiometric CNG engine still needs a three-way catalyst — and unburnt methane is a strong greenhouse gas that is hard to oxidise, so methane slip has to be controlled.

  5. 5.What happens if a vehicle designed for gasoline is run on LPG without proper conversion?Application

    It cannot run at all: LPG is stored as a pressurised liquid and must be delivered through a dedicated tank, shut-off valves, a vaporiser-regulator and a gas mixer or injectors, none of which a petrol car has. A proper conversion adds this hardware and recalibrates fuelling, because LPG's stoichiometric air–fuel ratio (about 15.5) and energy per litre differ from petrol, and usually adjusts spark timing; knock is not the issue since LPG has a higher octane number than petrol. Poorly done conversions cause lean misfire, backfire into the intake, valve-seat wear and gas leaks, which is why only approved kits fitted by authorised centres are allowed.

  6. 6.How does the energy content of hydrogen compare to that of CNG and LPG?Application

    Hydrogen has a higher energy content per kilogram compared to CNG and LPG. However, due to its low density, it requires high-pressure storage or liquefaction to be practical for vehicle use. This makes hydrogen storage more complex and costly compared to CNG and LPG, which are easier to store and handle.

  7. 7.If a vehicle's engine efficiency is 30% and it uses 2 kg of hydrogen, calculate the useful energy output in megajoules (MJ). Assume the energy content of hydrogen is 120 MJ/kg.Numerical

    The total energy content of 2 kg of hydrogen is 2 kg × 120 MJ/kg = 240 MJ. With an engine efficiency of 30%, the useful energy output is 30% of 240 MJ, which is 0.3 × 240 MJ = 72 MJ.

  8. 8.What are the main challenges associated with the use of hydrogen as a fuel in vehicles?Application

    The main challenges include the storage and transportation of hydrogen, as it requires high-pressure tanks or cryogenic temperatures. Additionally, the production of hydrogen is energy-intensive and often relies on fossil fuels, which can negate some environmental benefits. Infrastructure for refueling is also limited compared to conventional fuels.

  9. 9.Explain why LPG vehicles might have a reduced driving range compared to gasoline vehicles.Application

    LPG has a lower energy density than gasoline, meaning that for the same volume, LPG contains less energy. As a result, vehicles running on LPG may have a reduced driving range unless they are equipped with larger fuel tanks to compensate for the lower energy content.

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